They're not. TOS has 6 movies, and two good ones (TVH and TUC)
TNG has 4 movies and two good ones (GEN and FC)
TNG has a BETTER ratio, if anything. And frankly, nothing in INS or NEM touches the cartoonishly forced plot events of TWOK or the juvenile idiocy of TFF (although Insurrection tries, on that second one).
I know everyone has their opinion, but this is just ridiculous.
Khan is considered the greatest Trek movie, and solid as a movie in general, and may in the long-run even find itself with the title over the 2009 movie.
TVH was hugely successful. And yes, TUC was also good.
As for TNG movies, only FC was a hit. Gen had only the notoriety of being the first, and for being a cross-over movie of TOS and TNG, which was at the same time it's claim to fame and it's undoing.
As was said, TOS had a bigger leap to make from TV to movie, so its advancement was more noticeable. TNG was higher quality on TV, and it ran longer on TV, and with the exception of FC, it had a hard time leaving its roots.
Though, I credit a big part of the success of TOS movies to Khan and "Space Seed." The trilogy - Wrath, Search and Voyage are really the highlights and highpoints of the TOS movie franchise. Search may be the weakest, but as the mid-point of the trilogy, at least we had a continuing narrative.
Once the story thread that started with Khan was gone, TOS movies really became rudderless, it seems, as exhibited so strongly by TFF.