Carnivore teeth aren't small because they couldn't or didn't evolve. They're small because individuals with unusually large teeth performed poorly when hunting their particular prey. Teeth and other major structures are almost always subject to trade-offs—a balance of opposing forces of selection. Bigger weapons may be better for killing prey, but they may also prevent an animal from catching prey in the first place.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 83