How do we know so much about the Bronze Age collapse and so little about the sea people that caused so much of it?

I'm sorry, you're starting from two false premises here. One, we do not know all that much about the Bronze Age Collapse - in fact, it is perfectly reasonable to doubt that it was a 'collapse' at all. Two, we do know a reasonable amount about the Sea Peoples (the plural is necessary - there were several of them!).

Here are some previous posts for your reading:

Here's an AMA with Eric Cline, author of 1177 BC: The Year Civilization Collapsed;
u/Bentresh examines the Collapse as experienced by the Egyptians and the Hittites; and since you'll run into the Sea Peoples, here's also a roundup of their previous posts on the Sea Peoples;
Still on the Sea Peoples, here's u/kookingpot examining who they were;
But there is reason to doubt the 'Collapse' narrative, as u/UndercoverClassicist examines in this post.