Maybe Patrick Stewart was busy with other jobs? Like films, theatre and other television... You make it sound like he's just sitting around waiting for the phone to ring from Rick Berman. Ok, that's almost certainly true for the rest of the TNG cast, but Stewart's not exactly been out of work since TNG finished.
Let's keep some timings in mind here --- what was going on during the 3rd season of DS9? Star Trek Generations the movie.
What was going on during the 6th/7th season of DS9? First Contact.
Its really not as though the Star Trek TNG actors weren't somewhat easily accessible. It doesn't mean they didn't have blocks, but they all had to have some level of accessibly time wise.
Contract wise could be a different story.