Because people like open worlds right now, and they make the most buck because of that.
Just look at the sales numbers between Skyward Sword and BOTW(or for a better comparison some of the Wii U ports) for example. Hell even Skyrim still sells big bucks literal decades after it was originally released.
I imagine in a few years it'll fade away as people get tired of it: same thing that happened to platformers, fps games, racing games, horror games, etc. When single genres get big like this people tend to get tired of the same 5 tropes appearing over and over.
And game companies don't have to appeal to everyone you know. Focusing on one genre makes much more sense financially speaking than making a bunch of games for every genre. It's a no brainer.