Good question. I've heard the answer but forgot. Something like, Mitt's father was born in Mexico of U.S. citizen parents so Mitt is ineligible.
My own grand father, of many generations ago, fled religious persecution from France around 1690 and came to the U.S. (Huguenot), hence, I'm not eligible either and furthermore, I believe France might owe me some reparations.
Mitt is ineligible because he's a liberal liar.
Well, one of the reasons is that he does not yet have 1142 delegates - but after California votes, he'll likely be close, but may still not have 1142 - maybe that is why some call him 'ineligible.'
GOP voters have to let their three state members of the RNC (that would be the Chairman of the state GOP party, and the 'National Committee Man' and the 'National Committee Woman') that they are quite unhappy with the performance of the RNC in this election, and demand to know if they are supporting what the RNC has done so far in this election.
If GOP voters don't hold their feet to the fire, then they can do anything and get away with it - with presumed support from their state GOP registered voters.