The reason the monorail has not been expanded since 1982 comes down to the almighty dollar - buses are cheaper and do not require the large up-front capital investment necessary to expand the monorail. Other factors mentioned are primarily spin, not so much because buses are an inappropriate transportation mode, but because Disney has become far too overreliant on buses, having failed to make appropriate investments in monorail or other transportation infrastructure. Buses will always have a place in Walt Disney World, but they should never have been the only solution.
Actually, this is false. If one train breaks down on the Epcot beam, for instance, the other three trains can run back and forth in 'shuttle' mode between the two stations. That doesn't help the people on the disabled monorail, but reliability has more to do with the design and maintenance of the trains. There is nothing about a monorail that makes it necessairily any more or less reliable than any other mode of transportation.
Again, buses should properly remain a part of WDW's transportation system even if the monorail were greatly expanded, but there is no reason to make people change trains. A monorail can make a direct, one-seat trip from resort to theme park (or other destination, such as the Marketplace) just as easily as a bus.
There is no reason a monorail has to be operated in a loop like a trainset around the Christmas tree, nor does switching beams necessairily have to be a time consuming, cumbersome process (switching would ideally be minimized anyway in a well designed network). A 'looped' system I probably wouldn't call impractical, but it would be very inefficient. Indeed, operating an expanded WDW monorail system more like a real railroad has enormous advantages.
That said. monorail expansion isn't happening. Disney is too cheap.