I got this one. The larger dinosaurs were dependent on larger trees and vegetation that takes longer to repopulate after the asteroid hit than their life cycles would allow. The growth cycle of a tree is many orders of magnitude slower then reptiles. So at first there would still be a lot of small babies of big (dead) dinosaurs to compete with the smaller dinosaurs (that evolved into birds?) from the short grasses that would emerge post fireball. The large bodied dinos would ultimately die when they got too big to consume enough of the smaller vegetation to allow them to exist. This is as good as example as I can think of off my head: You could, for example, try to reintroduce a particular species of extinct locust that used to exist until the late 1800's and it wouldn't do any good. They only bred in the deepest ruts of the prairies before the vast buffalo herds got slaughtered (Arizona history books mention that were all wearing hoodies). You could theoretically release the first "batch" of artificially created/cloned buffalo locusts into the wild, but when it came to the point in their life cycle to breed, they wouldn't have anyplace to lay their eggs. No herds of buffalo. No eight foot deep ruts in the prairie worn down over eons for them to lay their eggs. Enjoy.