Why do most TDMs or whatever with the Class system have similar classes to Team Fortress 2, such as Boss battles.
Mainly because TF2 has most of the general ideas for classes. Fast weaker class, slower stronger class, so on and so forth. Their classes cover most grounds, so its just easier and simple to use the basis of what TF2 classes have, and harder to make classes that may stray from the norm (mainly due to the lack of ideas).