Why are the films based on the original series better than the TNG movies? It's all in the source material.
No, that's not it at all. If that were true, then the TNG films would have been a gazillion times better than the TOS films.
The TOS films got lucky because they had better writers and directors, and the TNG films got the short end of the stick with poor writers and bad direction (with the exception of Frakes who did a terrific job directing FC).
This inequality should sum things up:
TNG show > TOS films > TNG films > TOS show
Just my opinion, of course.