The primary cause is people not completing antibiotic treatments: a lot stop when they lose their symptoms, rather than when the doctor says. That way more bacteria survives, and can develop. If people would continue, there'll be less resistant bacteria.
However, radiation treatments will prove quite successful by 2022, or so. They eradicate all foreign organisms, and the body survives. After that, all it takes is the replacement of more beneficial bacteria that got killed too.