I’d like to think Lava bending is only for those who can (or has the potential) to bend both fire and earth.
Before Ghazan, the only Lava benders known were the avatars who, of course, can bend both fire and earth.
Ghazan’s origins are unknown, but we do know that Bolin’s dad was a fire bender (seeing as how Mako is a fire bender). So Bolin has both the genes for earth and fire bending.
And maybe the reason why there hasn’t been a non-avatar lava bender before is because the kingdoms were separated, so there couldn’t be a earth-fire marriage set up until the Republic city’s establishment.