As a producer, Colin Trevorrow certainly could have directed Fallen Kingdom if he’d wanted to, but instead he let somebody else do it.
When it comes to modern franchises, we generally see directors staying with successful franchises for the sequels.
The first two Thor movies had different directors, but following Taika Waititi’s success with Thor: Ragnarok, he was locked in for the next one as well.
Patty Jenkins has directed both Wonder Woman movies so far, and she’s already signed for the third.