No reason, it's just that there were more Rangies out there earlier.... basically, the Rangie came out in '70 (came to the US in '87), whereas the Disco came out in '89 (came to the US in '94) so, Rangies have been used longer. The chassis main rails, etc., are mostly the same. You're still going to have to mount the outriggers for the bulkhead, add a rear crossmember and fab some other mounts for attaching the rear tub to the chassis, etc.

Go for it!