The main reason to switch to SaaS is that it’s less of your responsibility anymore. The decision is made mainly not because of technical but legal or budget reasons.
It's not just learning how to secure it once, it's constantly watching for announcements regarding new vulnerabilities and being able to patch at short notice or being able to pull the infrastructure offline if you can't patch right away.
The world is a different place now with what virtually amounts to criminal companies trying to find every vulnerability that allows them to get into your system and either holding your data for ransom, extracting it for their own uses, or both. Even if you really do want to employ someone solely to stay on top of patching and watching for vulnerabilities, it's safer and often cheaper to let one of the big companies host your data.
people really freak out when it goes down or you screw it up. i’m willing to roll my own of most things, but I don’t want to roll my own mail server or phone server.
these things are just so specialized and publically exposed and the penalties are too severe if you fuck it up or lose data.
lobby and corruption. But i repeat myself.