You're comparing apples and oranges. The trains were and remain a ride. They aren't hauling the loads they did when they were actual working locomotives. Also, key word: locomotives. The locomotives are the only pieces of the railroad that are "hi-tech". with the speaker system and lights essentially being all that's "advanced" in the passenger cars.
The monorails, however, are a working transport system. Further, as rmwebs pointed out, the monorails involve a great deal of fiberglass, which isn't as strong as the steel that is in the locomotives. Sure, the monorails can be completely rebuilt via maintenance, but unlike the railroad, it's not just the locomotive, a locomotive using pretty simple, pre-electronic age technology as the core of what makes it move. But the older the monorails get, the more work needs to go into them, and the law of diminishing returns rears its head. There's a reason we don't still have busses from the 1950s in modern cities.