I agree that the 6 TOS movies on average are better than the 4 TNG movies on average. But if you remove Insurrection (which was fine) and Nemesis (which had a lot of flaws) from the mix, then that is no longer the case - so the question really becomes a narrower 'why is Instruction only okay and what went wrong with Nemesis?', rather than the broader 'TOS vs TNG' way you put it. If they stopped after First Contact, people would rave about the quality of the TNG movies.
Another way of looking at it is that if you alternatively removed TWOK (written and directed by Nicholas Meyer), TVH (written by Nicholas Meyer) and TUC (written and directed by Nicholas Meyer) from the TOS list then the comparison would also no longer be clear cut. In which case you could also phrase your question as 'why was Nicholas Meyer so good at making Star Trek movies?'