Had bacteremia (sinuses), but lots of people have sinus problems that don't end up in the blood, why couldn't my body keep it from going into blood?

No way to know here. In the limited amount of space we have, there' no way to know for sure. Consider HealthTap Prime or talking to your doctor to go over details. Immune system problems, frequent infections, surgery, some other anatomical problem (some are born without a sinus!) could do it. But in general, you're chasing your tail trying to figure it out. Importn thing is u got treated. Hopefully, you're healthy now.