When Epcot opened, they had double decker buses that went around World Showcase. IIRC they stopped at every other country. My parents loved to get up on the top level and just ride around and around, since you didn’t have to get off at any particular stop. They also had two of the Friendship boats, one on each side, instead of the single one they run now. Between all the festival booths and the increased crowds, the buses wouldn’t be practical or safe anymore.
At MK they used to have more transportation on Main Street throughout the day, with more double decker buses, the trolley, and fire engines. There was also the Skyway, which ran from the back corner of Fantasyland where the Tangled bathrooms are over to the front corner of Tomorrowland near CoP above the restrooms there. That was a good way to get from one side of the park to the other. They did add the third station to the railroad (the one in Fantasyland/Circus was not one of the original stations) so that helped move people around the park.
I think the parks are simply too busy for most types of transportation to be operated safely. People just don’t watch where they are going, so if you were going to add transportation it would have to be separated from the pedestrians. I have often wondered if they would consider bringing back the second Friendship boat at Epcot, at least during the busy seasons.
