Boses have traffic. People don't like traffic. Trains don't have traffic.
It's not a common occurance, but it happens often enough that people who use trains regularly can remember a time it happened to them.
(I tend to like trains, but I use buses and coaches more often because price and convenience.)
Buses confine you to a cramped seat and if you're lucky there's a tiny bathroom on board.
Some buses have WIFI.
Trains have traffic. It's baked into their schedules, so it can never improve based on variations in usage.