Also keep in mind... Epcot was built in 1982. What resorts existed back in 1982? The Contemporary & Polynesian, as well as Fort Wilderness and the Golf Resort. So, the monorail to Epcot made sense at the time. The only busses they needed was to shuttle from the Golf Resort & FW to TTC, and a FW Internal shuttle.
Now, off the top of my head, I can't think of exact dates, but when the Disney/MGM Studios was built (On a different area of property, mind you) Disney had already built (or was planning to build) Caribbean Beach, Swan & Dolphin, and Hotel Plaza Blvd. In addition, Grand Floridian was added to the Monorail loop, and Wilderness Lodge was built soon thereafter. And of course, Yacht, Beach & Boardwalk came along, as well as Old Key West, and Port Orleans and Dixie Landings. In addition, Along came Pleasure Island as well as other Downtown Disney changes.
Like I said, not sure of exact dates, but Im pretty sure those were all within 85-95. (Can someone give me exact dates?) So as you can see, between 1985 and 1995... Walt Disney World saw a huge growth spurt.
My point with this, is that when Epcot was built... it was a single location to a single location. (TTC to Epcot) When MGM was built, there were dozens more locations to service. A monorail would not be logistically feasible.