Because even if it is just a maintenance issue (which as others point out is weird for a a permanent part on a 10 week old plane that shouldn’t need maintenance on this part ever), Boeing and the FAA have completely shot their credibility over the 737.
Both in terms of original certification and also doubling down on “no issue planes are fine” until it became incontrovertible that the planes were the problem and then all the collusion and shadiness about how the 737 came to be came out.
So with that history, even if it would be an overreaction, rebuilding trust would look like immediately grounding in a situation like this out of an abundance of caution. Btw the 737 has had other maintenance problems crop up since they put it back in service since the fleet was grounded to retrofit the part that was needed to fix the original fatal-causing defect. Honestly this just looks like an unsafe plane top to bottom that could sink Boeing (except they’re not allowed to sink because they’re critical partners to the national defense industry)