Simple: every other race in Middle Earth has rings, why do Hobbits and Orcs not?
Sauron didn't even know that Hobbits even existed until very late in the Third Age:
'Yes, alas! through him the Enemy has learned that the One has been found again. He knows where Isildur fell. He knows where Gollum found his ring. He knows that it is a Great Ring, for it gave long life. He knows that it is not one of the Three, for they have never been lost, and they endure no evil. He knows that it is not one of the Seven, or the Nine, for they are accounted for. He knows that it is the One. And he has at last heard, I think, of hobbits and the Shire.' (Shadow of the Past)
The Rings of Power, however, date to the mid Second Age, and so Sauron just would have been unable to give any Rings to Hobbits.
As for the Orcs:
But Sauron gathered into his hands all the remaining Rings of Power; and he dealt them out to the other peoples of Middle-earth, hoping thus to bring under his sway all those that desired secret power beyond the measure of their kind. (Of the Rings of Power and the Third Age)
There is no need, because the Orcs are already under his domination.
Hobbits aren't a separate "species". They're a subdivision of men, in a similar way to that which the Druedain are a subtype, but very different of men.
"It is plain indeed that in spite of later estrangement Hobbits are relatives of ours: far nearer to us than Elves, or even than Dwarves. Of old they spoke the languages of Men, after their own fashion, and liked and disliked much the same things as Men did. "
LOTR - Prologue 1. Concerning Hobbits
So Hobbits are relatives of ours, who underwent later estrangement. They're not a completely separate species, like Elves and Dwarves.
Other races (even just "good" races) don't have rings either (eg : Ents, skin-changers etc).
In brief, the Elves made the rings, gave 3 to the keepers, 1 to the King of Moria, then Sauron stole the rest and handed them out to Mannish kings and heroes to corrupt them. He needed powerful, easily swayed Kings for this purpose. Hobbits didn't fit the bill.
There are no rings made for dwarves and men
The rings weren't designed to be '3 for elves, 7 for dwarves, 9 for men' - that distribution occurred only after the rings were made.
There were many rings of power made by elven lords, 19 of them considered the greatest but including also many lesser rings. They were made by elves for elves until the treachery of Sauron was revealed by making the One ring that dominates the others, and elves discarding all of them except the three that were made in secrecy from Sauron. There is no reason whatsoever to suppose that the elves made the rings with a plan to give them to dwarves, this was done by Sauron after he obtained the corrupted rings.
The distribution between dwarves and men was probably made ring-by-ring, wearer-by-wearer according to whatever plans Sauron had - and the nice poem comes only afterwards. One could also speculate that Sauron gave some of the lesser rings of power for lesser leaders of men, orcs and others.
Short answer: Because orcs and hobbits are not actually separate races, but subtypes of elves and humans, respectively, and neither group presented a military threat to Sauron's power. Though it could be argued that the orcs did get a ring, the one ring in fact, since Sauron is their leader even if he is not an orc himself.
Orcs are not a separate race but a corrupted form of elves created by Sauron's boss, Melkor (who the elves later renamed Morgoth).
"Yet this is held true by the wise of Eressea, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken, were put there in prison, and by slow arts of cruelty were corrupted and enlaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the Elves, of whom they were afterwards the bitterest of foes." (The Silmarillion - Of the Coming of the Elves and the Captivity of Melkor)
For those who haven't read the Silmarillion or just plain forgot (like me), the Quendi is the name the elves gave to themselves shortly after they came into being.
As mentioned in the answers by WOPR and Darth Satan, hobbits are also not a separate race but a human subtype, and were not known to Sauron at the time the rings were forged.
Another way to look at the question would be that Sauron wasn't trying to give rings to every race, but to every major political leader that might oppose him. Neither orcs nor hobbits nor any other group which didn't get a ring had anyone in that category, either because they lacked military might or were already allies. That being said, Tolkien never was terribly clear on how the rings functioned, so it's probably best not to make any assumptions as to Sauron's intentions.
I don’t think it’s explained in canon.
It was possibly because the rings were given to rulers, while the Hobbits had no rulers to give the rings to.
Or more likely, it was that Sauron decided that the Hobbits weren’t worth the effort of enslaving directly, and instead focused his efforts on the Elves, Dwarves and Men to put his plan into action more quickly.
In comparison to, say, Elves or Men or Dwarves, Hobbits didn’t have much to offer in terms of world domination and subjugation. Despite the Gondorian rumours of the Hobbits being small but doughty warriors, they had nothing noteworthy in terms of combat skills. Doubtless they could till the earth, grow crops/pipeweed, and brew ale, which in terms of logistics might have come in handy, though. The fact that it was wise to be wary should a hobbit stoop for a stone, had not, I think, reached the Crebain of Saruman or the agents of Sauron. Even if they were a force to be reckoned with with a stone, you would need a lot of Hobbits with a lot of stones throwing them very quickly to make a difference at, for example, the battle of the Pellennor fields.
So No: They clearly weren’t considered worthy of receiving a Ring of Power.
This is all according to what I have read from the Simarillion (Tolkein's telling of the world's creation and first ages) and learned from the various answers from this site.
According to the Simarillion, Eru Iluvatar (the creator of the world of Arda} created Elves first then Man. The Dwarves were created by a different god but still blessed with life from the "good" gods. On the other hand; The Orcs were made by Melkor (bad god) by cursing and deforming a fragment of the Elves (he was cursed by Eru to never be able to "create life".) Hobbits are distant relatives of Man, not an entirely different species of beings.
So therefore, Sauron (Melkor's lieutenant and successor), would want to subjugate the races created by Eru. So he convinces and watches the Elves' greatest smith Celebrimbor to make the rings of power. 7 to the dwarf lords, and 9 to men. The 3 forged by the Elves were done by Cele after Sauron had left with the other 16. Sauron then forges the One Ring in secret using the techniques he had learned from Cele plus pouring his will and malice into it.
In closing: Sauron doesn't need to create rings for the Orcs because they are already subservient from ages when his master warped them from already existing elves. As far as Hobbits; basically they are an offshoot of man and therefore would be under his subjugation by default due to man being under his control as well.