He left pop still relevant, but he did release an album after those, in 2001 but it was classical.
Since then its been some live or compilations.
He knew when it was time, pop had moved on... as it always does. He had a long run, but the window for his music was closing and he didn’t need the money.
Not sure what he’s said on the matter, but that’s how I read his leaving pop.
I don’t really consider that Classical album a Billy Joel album. He didn’t play on it (and based on what I’ve heard had some ‘help’ in writing it). His back catalog is still hugely popular. He hasn’t really faded away he just didn’t diminish his catalog by releasing an endless stream of forgettable albums. Those compilations and live albums are record label releases that he has no part in. He’s not giving them new music so they keep repackaging his older music (because it remains so popular).