I used to work for Metrolink, and can see where you're coming from. However, slightly bizarrely, although we had plenty of ex-bus drivers, we didn't have many truck drivers - at least, not long-distance truckers. There were a fair few ex-home delivery drivers though.
I can't think of any reason why this should be the case - bus and truck driving probably requires a similar set of skills after all, though I don't know how many truck drivers apply - I can imagine those doing long distance may struggle to adapt to a rigid system of duty times and routes where previously they had an element of freedom within the bounds of company policies and the Road Traffic Act.