It's just that many people want to be able to use the software by doing modifications to it for their secret sauce. And they don't want to share the sauce. AGPL forces you to share it, even if you don't distribute the software: as long as you make a service with your modified version, you are legally entitled to share the modifications.

Of course, it's almost impossible to prove IRL, but still, a lot of corporations have problems with that. They want the free open source project to make them earn money, no price to pay.