I can remember when Cardiff was the larger airport.
Sadly, it’s been left behind.
Bristol has had more extensions than the Allbright pub. It’s still a nasty cramped airport, though - particularly the Border Force Area, which is one big bottleneck.
It’s succeeded because they’ve wrung every last penny out of it.
As another poster said, it has a more affluent catchment, which means it’s not as dependent on the charter market and it's also closer to the rest of the UK than Cardiff, which is always going to be a problem.
As usual, money talks.