What was wrong with monorails? From this answer:


  Then there's the whole problem of parts which are simply not in production any longer, and for which the cost of reproducing them would offset the potential benefits of reissuing sets. (In other words, don't expect monorails to be reissued ever).


And this comment:


  The cost for producing an old element again is presumably lower than the cost to develop a new element altogether, as long as the molds still exist (monorail parts molds are rumoured to have been destroyed, for example)


I'm getting the impression from these comments that there were issues with the monorail sets.  What problems did they have?  Were they too expensive to produce?  Why would LEGO destroy their part molds?  Why wouldn't they re-issue monorail sets?
For the molds: An injection mold is made for a specific injection molding machine. I very much doubt Lego is still using the same machines as they did 20 years ago. So to keep the molds, they would have to keep the old machines, including the service teams and infrastructure. 

Also, machine parts are rarely made out of stainless steel, some old Lego ones clearly weren't. Stainless steel is just too bad at conducting heat, which is needed in an injection molding machine. In order to keep the molds for 20 years, they would have to keep them in shape, which means additional effort. 

With regard to play value, and here it is becoming subjective, the monorail never added up. Monorail trains are only 4 wide structurally, while trains are 6 wide. Making monorail 6 wide would end up in a very unstable situation when manipulating the inside / load whatever. A robust, universal toy, they weren't.