Boils down to money. A brand new type certificate for a big passenger jet is in the hundreds of millions of dollars in time and money. Therefore you can justify spending quite a lot on lobbying (actual and metaphorical) to get away with reusing one.
Edit: And the airlines will lobby on your behalf too. They don't want to have to certify pilots on a new type, or have yet another split of who can fly what.
Edit: And the airlines will lobby on your behalf too. They don't want to have to certify pilots on a new type, or have yet another split of who can fly what.