There wasn't one thing that killed the TNG film franchise, but Insurrection did not help. It was a rather bland film that would have been better suited as a TNG tv episode rather than a major motion picture. There was nothing cinematic about it.
Beyond being a bland film it was quite frankly a terrible film that imho is the worst of all the ST films thus far.
Also, at the point that Insurection was released the Star Trek franchise was suffering severe over saturation. There just wasn't anything special enough about this story that would make fans run out to the theatre to see it when they could get ST for free at home.
The TNG films in general with the possible exception of "First Contact" were not big/epic enough to carry a film franchise. Instead of kicking off the film franchise with the depressing death obsessed "Generations" TPTB should have kicked the TNG film franchise off with a fun action adventure film that involved the TOS meeting the TNG crew and a proper passing the torch in a manner that would have fired up the fanbase...Instead we got a totally forced cameo by William Shatner where Kirk is killed off for no reason in particular. As a matter of fact Generations didn't bring the fanbase together it did the exact opposite by killng off Kirk and minimizing TOS fans (It's no coincedence that when the series needed to be saved they went back to Kirk/Spock) Generations was too depressing and polarizing a film to kick off the TNG film franchise.
First Contact was a step in the right direction, but Insurrection and Nemesis with their corny story lines (B4, Data, Picard, Worf singing etc) were just horrible and the final nail in the coffin of the series.
I hope the TNG series gets a better ending someday. Perhaps a tv mini series or film.