Because they have a differently shaped heel. I'm not 100% sure, but I think you can put a Tele neck on a Strat body (or vice versa), but there will be a small gap showing (somebody correct me if I'm wrong). I know the old Warmoth site explained this in detail (I haven't searched the new site for this info).
Also, Mustang necks are 24" scale, so they won't match up unless you move the bridge.