The test screenings were so poor they were forced to reshoot the end. Even with the revised ending, audience reviews were lukewarm.
According to the Wikipedia article on the movie, it was shot in just 56 days! Production was so rushed, Frakes and Burton wore Avery Brooks and Colm Meaneys costumes from DS9. Apparently, filming on Generations was happening concurrent with filming on All Good Things.
Ron Moore: I think that is probably a valid point. If you listen to the commentary track on the Generations DVD, Brannon [Braga] and I talk pretty openly about our dissatisfaction with Generations and the reasons why it went south and the reasons we are not happy with it. I kind of feel it was a missed opportunity and a movie that just didn’t come together.
I really think that with more development time, this movie could have been great.
Yup. Everyone was pissed at how Kirk was "shot in the back". With the scene available to view, now more than a select group of audiences in test screenings can see it. There's a tangible sense of tragedy and palpable gut-punch in his demise in the deleted scene, with Soran accomplishing what Khan could not (ouch!!), which doesn't exist at all in what was filmed and released instead - which is corny to the max. Kirk was to be killed off in a surprise and unexpected moment, since TNG was the future and it's too dorky fairytale to have Kirk survive. (Yep! Decades before Luke Skywalker, James Kirk was killed off for dramatic effect.)