I think The Last Stand only did so well opening weekend because it was riding on the coattails of X2, which was a big success and generally considered to be the best film in the trilogy. People were chomping at the bit for the third installment because they were invested in the story and the characters.

Then Brett Ratner came along and f'd all that up to the point that afterward, I had friends saying they'd never see another superhero movie again. (None held to it, but the point is they were almost irrationally disgusted with X3). Cut to five years and 20 comic book films later, and I think a lot of casual fans of the genre are burned out, and prefer to wait to see what the general consensus on a film of this type is before they plunk down their money. Suffice to say that The Last Stand did 67% less business its second weekend. The Last Stand probably turned as many people off from seeing another X-Men film as X1 and X2 did the opposite.

In short: Ratner's turd did well at first because X2 was awesome. That is all.