I think because it was the first time music had been used onboard at Space Mountain.
And for the record, I definitely prefer the D. Dale soundtrack to the current one.
The '05 promotion was for Rockin' Space Mountain, where both Space Mountain and California Screamin' would have daytime and nighttime versions of their soundtracks. At dusk the change would happen and the more "rockin" version of the music would be switched on for both rides. It was a gimmick and promotion that failed spectacularly. I seem to remember the band called Red Hot Chili Peppers did the soundtrack for California Screamin', but it wasn't very good and their peers in the music industry and hardcore fans made fun of them for doing such a hokey thing for Disneyland. The memory is a bit misty, but I seem to remember the Peppers band had to issue a statement saying their work for Disneyland was a mistake that they apologize for.
The signage for Space Mountain was visible during the day, where the sign was going to change to relabel the ride Rockit Space Mountain after sundown. I think they decided that would muddle the "Rockin'" message, so they eventually just slapped a "Rockin" sign on top of it. But the Rockit Mountain name was kind of clever. When the sun hit the sign at the right angle the "Rockit" wording was visible for several years.
Again, memory is misty here without researching, but the end result was so bad they ended the promotion sooner than later. WDI was left with their tail between their legs after wasting a bunch of money on it, which is what WDI loves to do. But at least this time, they couldn't send Joe Rohde and his groupies to live at a foreign Ritz-Carlton for a month for "cultural research" on this roller coaster.