According to https://forum.brickset.com/discussion/comment/57906/#Comment_57906 :
The reason LEGO has stopped making monorail is threefold,
One, the motors and track molds ran out and would be very expensive to replace.
Two, no monorail set ever made any money, in fact despite trying several different types at high price points they all cost more to produce then they were sold for.
Three, kids these days don't see them as exciting, they are from theme parks or airports, not a futuristic thing at all, in fact pretty boring.
On the subject of molds: according to https://bricknerd.com/home/lego-monorail-bring-it-back-or-build-your-own-7-6-21 ,
The molds and the motor aren’t available anymore because they were produced by a contractor that no longer exists.
See also the question here: What was wrong with monorails?