Actually, a fixed rail system is the most efficient method for a place like WDW high concentrations of people moving between fixed places on known schedule. Busses are great for low density routes with even demand throughout the day. Busses are extremely inefficient with dealing with issues like park closing the only way to expand capacity is to add a bunch of busses. The rest of the time all those extra busses sit in the back and rust. A train can haul around its "extra" capacity all day for a minimal cost, and it's always available when needed.
The problem comes in the initial cost. Unlike busses or even light rail no one makes monorail systems on an assembly line. Each one is a custom job and that really drives up the cost. If the corporate will is there, transportation can still happen: Tokyo
Disneyland opened up brand new monorail system and Hong Kong Disneyland opened an special line to connect to the Hong Kong subway system (the trains have Mickey shaped windows and Disney art inside).
At WDW, however, the current business model is going prohibit any major changes. Any new project has to make money all by itself considerations of improvements to the overall resort are ignored. They had considered charging for monorail transportation, as Tokyo does, but Disney simply wants to spend the money on other things, like bailing out ABC Family.
There have also been long standing whispers that WDW's bus system will be outsourced to Mears just like the airport and cruise line busses are. If, or when, this will happen remains speculation at this point.