In TNG the enterprise weapons and torpedos did nothing to the borg and the borg were able to adapt right away to the star fleet weapons, or so my memory tells me, but yet in first contact the star fleet weapons just tear the cube to pieces how?
The television series was able to do more thoughtful and creative things. they realized the real terror would be weapons being more or less useless against the borg, and thus as the Enterprise and other ships fired on the borg, they had no success (usually) and no real explosion.
But the movie needed to be BIG BIG BIG, and they were interested in making it something "a general audience" can enjoy, which means DUMB IT DOWN, DUMB IT DOWN. Star Trek fans would pay to see it no matter what, so who cares about making a movie for them? If a general audience saw the weapons doing nothing, or fizzling out on the surface of the cube, they'd be like, "Duhhhhhh....why no go boom? Need go boom and fire and 'splosions!" But with giant loud blow-'em-up torpedos, that general audience can hoot and fist pump and so forth.
Why could they blow it up right away, after Captain Picard magically remembers the special spot to fire on to blow it up? Because the plot needed it. They weren't making a movie about an endless space battle. They needed the cube gone. So what if Picard remembers a magic weak spot? No one is going to detect or blow up the tiny sphere that exits, apparently, not even as it flies through a fleet of starships. Have everyone fire on the same spot and blow the thing up.