My only guess here would be the evolution of ride technology.
Oblivion was brand new technology and a prototype when built and no other dive machines had been constructed at the time. When Griffon opened, at least several other dive coasters had opened, not to mention it was no longer brand spanking new technology either.
Another thing to mention is the big hole they dug for oblivion wouldn't have been cheap either, especially since much of the drop is actually beneath ground anyway.