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The One Ring abandoned Gollum, expecting to be found by an orc but when Bilbo found it, why didn't it then abandon Bilbo?
It could have easily fallen off when he was squeezing through the gate after following Gollum out of the cave and there were goblins right there.
You say that it could have "easily fallen off" but that's exactly what it did. As you can see from the passage below, it fell off at the worst possible moment, nearly resulting in his capture by the goblins. Unfortunately (for the Ring) it fell back into Bilbo's pocket rather than onto the floor:
Bilbo blinked, and then suddenly he saw the goblins: goblins in full armour with drawn swords sitting just inside the door, and watching it with wide eyes, and watching the passage that led to it. They were aroused, alert, ready for anything.
They saw him sooner than he saw them. Yes, they saw him. Whether it was an accident, or a last trick of the ring before it took a new master, it was not on his finger. With yells of delight the goblins rushed upon him.
A pang of fear and loss, like an echo of Gollum's misery, smote Bilbo, and forgetting even to draw his sword he struck his hands into his pockets. And- there was the ring still, in his left pocket, and it slipped on his finger. The goblins stopped short. They could not see a sign of him. He had vanished. They yelled twice as loud as before, but not so delightedly.The Hobbit - Riddles in the Dark
In short, it was trying to escape from him the whole time he had it. Gandalf notes that it would periodically shrink or expand (as well as changing weight) in an attempt to fall off of his finger.
'Bilbo knew no more than he told you, I am sure,' said Gandalf. 'He would certainly never have passed on to you anything that he thought would be a danger, even though I promised to look after you. He thought the ring was very beautiful, and very useful at need; and if anything was wrong or queer it was himself. He said it was "growing on his mind". and he was always worrying about it; but he did not suspect that the ring itself was to blame. Though he had found out that the thing needed looking after; it did not seem always of the same size or weight; it shrank or expanded in an odd way, and might suddenly slip off a finger where it once had been tight.' FOTR - The Shadow of the Past
Because the Ring wasn't specifically trying to be found by an Orc.
That's just a rather flippant, throwaway comment that Frodo makes ("Wouldn't an Orc have suited it better?") The Ring's purpose in abandoning Gollum was to get out of his cave and away from him, because he had exhausted his usefulness and the Ring needed to start finding a way to get back to Sauron.
This is explained by Gandalf in Shadow of the Past:
It could make no further use of him: he was too small and mean; and as long as it stayed with him he would never leave his deep pool again. So now, when its master was awake once more and sending out his dark thought from Mirkwood, it abandoned Gollum.
It didn't really matter at the time who found it - an Orc or anyone else; the Ring's motive was just to get away from Gollum.
The Shire, on the other hand, was a suitable enough location and (bearing in mind the points made in Richard's answer) was somewhere from where the Ring could begin working it's way back to Sauron.
Gandalf says that Bilbo was meant to stumble across the ring & Frodo was meant to bear it which i interpreted as Eru playing a part in just as he did when Gollum fell to his death
