Because either a) they're vandals, b) they were irritated at being unable to steal the rest of it, or c) they're crackheads & even the couple of quid one might get for clearly thieved handlebars is better than nothing. Or d) all of the above.

Though *actually* I suspect not vandals, as the wheels were still intact & not kicked in.

The cost of replacing everything they took is around 120quid (inc small labour charge), so presumably the second-hand parts are worth *something* :-/