TNG was on air for seven seasons, amassed a big fanbase of its own and was about to go into feature films, surely the series had enough weight behind it to do its first film without a crossover.
In order to work, the movie had to bring in a non-TNG audience.
I suppose the thinking by the studio suits was that movies and television are two distinct beasts with two potentially distinct demographics, and that while TNG had undoubtedly proven itself to be a major success as part of the Star Trek phenomenon (the fact that they even green-lighted TNG's move to the big screen in the first place was proof enough of that
), there were still those at Paramount who felt they were somehow 'unproven' as a big box office draw. Hence having a crossover with the previous movies was deemed essential as a way of hedging their bets.