I assume it's all about the money. If a vehicle's resale value drops too much below the value of the major parts, more end up being sold in pieces. Some never get to that point -- you don't see many Ferraris in wrecking yards.
Good point. It also depends a lot on parts availability. If you have a car or bike that's still loved but key parts can't be replaced, all of a sudden it's real easy to consider cutting one up to save your own.