Torrenting and child porn, I would guess. I remember the slow transition from people using open networks to securing the shit out of them, and there was this big fear that someone could use your network to download copyright-protected and/or illegal material, and it would be tied to your IP.
Indeed, 10-15 years ago in almost any area with a population you could easily find an open network to get onto the Internet when you wanted to. I could check email and lookup some quick things even while riding public transit, without needing mobile data. There was also a grassroots movement of sorts to "share your WiFi", and even a well-known security professional opened his: https://www.schneier.com/blog/archives/2008/01/my_open_wirel....
Now there's almost none of those left, and what places do advertise "free WiFi" are captive/login portals. It was more free and open back then, I actually quite miss those days...
I suspect it was the ISPs who made the decision, not the manufacturers. Removing the common excuse of "my network is open, who knows who did it!" for torrenting may be the reason. Another could be to make open/easily accessible networks rarer as they started selling Wi-Fi via the provided routers.
could be ISPs trying to improve user security generally to prevent lots of their customers becoming DDOS zombies resulting in more contention & customer complaints.
