Because it was a TNG movie, thus TNG characters must star. That's the reason. Plain and simple. People wanna see Worf in battle, not Sisko.
Now don't get me wrong, I like Sisko as a captain but Worf is simply bad ***. And quite frankly "Perhaps today is a good day to die!" couldn't of been said by any other person but Worf.
Having Sisko would of been too complicated and wouldn't serve the story. Remember that First Contact is a movie and the first thing writers want is a story that works for the audience. Sisko in the movie would of been sugar coating and "let's please the DS9 fans for no reason."
Sorry if that sounds harsh. Shirley you must allow me to speak my mind!
Don't call me "Shirley."
"WHERE'S THE KA-BOOM? THERE'S SUPPOSED TO BE AN EARTH SHATTERING KA-BOOM!!!" -- Marvin the Martian