The official reason is that California had required Diablo Canyon to rebuild their cooling system, to minimize continued impact on reef habitat. PG&E solicited

The official reason is that California had required Diablo Canyon to rebuild their cooling system, to minimize continued impact on reef habitat. PG&E solicited bid proposals for a new cooling system, received some obscene number, and subsequently made the formal decision not to seek renewal of the relevant licensing. But everybody knows the real reason is because of anti-nuclear activist pressure. If PG&E was strongly motivated to keep Diablo Canyon open, they probably could have, but they'd deplete whatever remains of their political capital. In the end it just wasn't worth the bother. They have bigger problems to deal with, and more important battles to fight. And they drew that conclusion before some of the more recent wildfire fiascoes.