If Tom Riddle Sr. and Tom Riddle's grandparents were Voldemort's last murders prior to Priori Incantatem in the graveyard in Goblet of Fire, then those murders were performed with Morphin Gaunt's wand, as Tom Riddle framed Morphin for the Riddles' murders. So those three victims would not emerge from Voldemort's wand in the graveyard. However, this doesn't explain why the Muggle tramp, Hepzibah Smith, or the Albanian peasant don't appear as well. (Moaning Myrtle having been killed by the basilisk)
The HP Wiki suggests that the screaming sounds emitted from Voldemort's wand during Priori Incantatem was the regurgitation of the Cruciatus Curse.
Conjecture: Perhaps Priori Incantatem is somehow tied to spells involving the human spirit -- the Unforgivables maybe. It's not like Voldemort's wand spat out Tarantellegra, Levicorpus, or the Bat Bogey Hex! Come to think of it -- why didn't it spit out the spell required to make a Horcrux? That spell would have come after a murder, and Horcruxes involve the human essence. Hmm. I have mixed feelings on it. Good question!
ETA: I found this explanation in Goblet of Fire:
I am guessing other such forms appeared... less recent victims of Voldemort’s wand ...’
‘An old man,’ Harry said, his throat still constricted. ‘Bertha Jorkins. And ...’
‘Your parents?’ said Dumbledore quietly.
‘Yes,’ said Harry.
Sirius’ grip on Harry’s shoulder was now so tight it was painful.
‘The last murders the wand performed,’ said Dumbledore, nodding. ‘In reverse order. More would have appeared, of course, had you maintained the connection.
So apparently when Harry broke the connection between the two wands, the other murder victims didn't have a chance to re-emerge from Voldemort's wand. This is from chapter 36, The Parting of the Ways. I also found the answer to my ghost question :)