So I am looking at getting an older Bay boat to start fishing Matagorda. Why is it the majority of bay boats have 150 hp or less motors even on 21'. I am I correct in assuming less daft/weight for shallow water fishing? And does a 21 foot boat do well enough with a 150?

You are correct. Most will run a low pitch 4 blade prop. That will give you the hole shot and load carrying capability, but you sacrifice high end speed. Most are willing to take this sacrifice as shallow hole shot, draft and running will be greater then speed.

That's good to know, thx