It's because a lot of them were culturally anti-copyleft, but had never read the licenses or the reasons behind them.
Copyleft is politically scary to some people, so they refuse to acknowledge it other than to call people zealots. Explicit licenses protect you. Open source spirits don't. Everyone should be honest: the only reason a lot of people prefer open source is because they want to preserve a rug-pull option. Then they get surprised with it's Amazon pulling the rug on them.
Disclosure: I work for Amazon, but I've been a copyleft advocate for a quarter century.
Indeed, copyleft can be politically scary. Especially when for-profit companies co-opt copyleft to drive licensing revenue by selling alternative license arrangements [1]. If all those who adopt copyleft licenses pledged to commitment to community-oriented GPL enforcement principles [2] I think that it would be a lot less scary. Unfortunately we've seen "copyleft trolls" that try to wield copyleft as a weapon, either for profit or to make other demands that are not helpful to the community.
Copyleft licenses are, indeed, protective licenses from my perspective. Or, they should be.
An aside, when it comes to "Amazon pulling the rug" -- what exact incident are you referring to?
I've been into free software since I was a kid, so all of this is very familiar to me. You've done a great job of breaking it down! Thanks for this and similar comments on this post. I hope they clear things up for some people. I feel that GPL (and AGPL, even more so) often does not get a fair shot, and that's a shame.
Copyleft is politically scary to some people, so they refuse to acknowledge it other than to call people zealots. Explicit licenses protect you. Open source spirits don't. Everyone should be honest: the only reason a lot of people prefer open source is because they want to preserve a rug-pull option. Then they get surprised with it's Amazon pulling the rug on them.