• Fixed-guideway services can run in a narrower right of way (or tunnel) at higher speeds without crashing.

• Capacity/bunching - A bunch of separate smaller vehicles is much worse. Passengers don't/can't distribute themselves efficiently and leads to heavy delays and varying loads between cars. This is part of why the Green Line is going to much longer vehicles on the Type 10's.

• A broken down bus in a narrow tunnel/right of way isn't really any easier to remove.

• You still can only get to it from the ends of that tunnel/corridor, so it's not like it can just pull over out of the way or have others go around it.

• A bus long enough to handle the capacity loads of a train, is not a bus that's going to be easy to just remove via city streets.

• If anything it's probably harder, because you still need to steer it.

• Operational costs - More operators needed. If you think full automation is possible, you're still talking a lot of duplication of guidance systems/sensors with more lower capacity vehicles.

And if you try to solve all those problems while not running "on" rails - your concept will eventually just turn into a rubber-tire metro, like you see in Montreal. They work fine and have some pros/cons, but there's nothing about them that's easier in the senses you're talking about.