Probably because trucks are industrial vehicles built to be driven millions of km*, and therefore are a much bigger investment, so the repair is more often "worth it" than for personal vehicles.

Also, one truck can have the cargo capacity of many cargo vans, and one bus in pendular movement has the capacity of many personal cars, so each truck/bus repair will have a greater impact.

*At least, that's the case for buses, according to a schoolbus driver when I was a kid.