~Could it be that with nothing to really to look at anymore (motorboats, skyway, Mission to Mars, American Sings, all closed before the PM) the People Mover got less popular?
~Could it also be that the executives were thinking "hey, we can save money on labor and maintenance by closing 3.5* more attractions and just re-opening one to replace it with"?
~Monorail......I submit that Paul Pressler was determined to eliminate all movement from Tomorrowland(or maybe it was all attractions that couldn't sell plush). He may have never actually closed down the Monorail but the fact that he did not build a station at the Hotel the Monorail goes thru speaks volumes to me....could it be that the PM was plenty popular enough still but because it took 4,000 people an hour off the walkways and out of the shops he needed for it to go bye bye?
~Ticket Books......ultimately, the PM, Keel Boats, Motor Boats, Skyway and many of the rest of the "minor" attractions may have been doomed when the Pay-One-Price admission replaced the books (people didn't want to "waste" their time on too many minor attractions when they have been forced to pay a premium price)
*I count the elevator to the Rocket Jets as a half attraction here.