I do wonder if 'mass market appeal' is a large part of it. FC clearly tapped into something that the other three TNG movies quite frankly didn't (GENS had the benefit of coming off the back of TNG ending and the crossover with Kirk, but internally it was arguably far too inwards looking to have true mass market appeal). NEM screwed itself over by advertising "A Generation's Final Journey...", subconsciously planting the seed into the minds of casual movie-goers that it was another naval-gazing exercise for the fans rather than a good movie in its own right. I'd actually say NEM had the potential to tap into the same broad appeal as FC, but as I say they fumbled the ball.
I think NEM never really had a chance. The franchise was clearly struggling by that point and suffering from fatigue and over-saturation. Combine that with the lousy marketing of NEM(I don't remember seeing ANY commercials for it in the weeks leading up to it), and you get a recipe for failure. Remember that even before word of mouth could take effect, NEM STILL opened up second at the box office to "maid in manhattan" of all movies. Even had NEM been better regarded, it would likely have made 60-70 million at most. There just wasn't the fanbase interest at that point.