I think there is also a question of Union players having different roles and skill sets i.e. a union prop is more about the scrums and do not have as much stamina as their league counterparts. Maybe a lock or union second row would work but little guarentee they would be able to successfully adapt.
I would not rule out a union player converting and being a success but it is more likely to be a Centre, wing, second row/flanker.
other issues are that there is more money where they are.