A combination of the ratings falling and the show being taken in a direction that many did not like (e.g. evil leapers, leaping into the lives of real people...)
Also, NBC was making some really dumb decisions around the time QL was cancelled (1993). Cheers had its swan song in '92, and the shows (along with Cheers) that made NBC the #1 network in the '80s---The Cosby Show, Night Court, The A-Team, St. Elsewhere, Hill Street Blues, Miami Vice, L.A. Law, Family Ties, The Facts of Life, and I believe others I'm forgetting now---were either long gone or on their last legs. The shows NBC used to replace the old ones (with the exception of a Friends here or a Seinfeld there) were nowhere as good. Apparently the network thought its success in the '80s would carry over to later decades. It was wrong.