A Question for Evolutionists on Reptile Teeth

Heterodontosaurids were small dinosaurs who lived from the late Triassic to the early Cretaceous, a period said to have lasted more than 100 million years. And they lived over a vast area of the earth, basically everywhere but Australia and Antarctica. They were members of a sub-order which contains some of the most common and best-known dinosaurs.

Given all of that, it is quite amazing that among all dinosaurs, and indeed reptiles, living and dead, only- this tiny group ever developed multiple types of teeth. They had three kinds of teeth to be exact, like humans and many other mammals. The earlier Dimetrodon had two kinds of teeth (hence the name), but was not a dinosaur, nor really a true reptile as were the archosaurs.  All reptiles, including dinosaurs, only have one kind of teeth in their mouths.

If macro- evolution is true, I wonder why the extremely useful trait of multiple teeth forms shows up only in this one obscure (yet widespread and long lasting) group? If all dinosaurs sprang from a common ancestor, and if some dinosaurs later became birds, why didn't some other groups develop this useful trait? Indeed why didn't multiple other reptiles? It makes no sense that evolution can produce such vast changes in form as we have seen in the fossil record, and as we can see today amongst the array of different reptiles on our planet, and yet the highly useful adaptation of multiple tooth forms came about once and only once. 

It seems like if reptiles can go from Turtles to T-Rex's to Chameleons to Snakes to Croc's to Cassowary's and tens of thousands of forms in between, then some of those creatures, besides that one obscure group, might have picked up the gigantic advantage of multiple tooth forms along the way.