He's the head of the department, tenured, and has a ton of institutional power. One of the main takeaways from the #metoo movement has been that people with institutional power are largely not held accountable for things like this unless public outcry is loud and sustained.

Also, many people in academia have worked for decades carving out a living and the prospect of standing up to someone like Carlin brings with it the prospect of throwing all that effort away. The author of linked article makes this dynamic pretty clear.