Why do people go with MIT over a GPL? Would have just releasing it under GPL in the first place also have prevented this?

Don't get me wrong, I have released libraries under MIT before because I have absolutely no problems with others using it even if they want to commercialize it, but I've also released things under GPL3.

Even AGPL would not have prevented this; it would just have required anyone else offering Bear as a service to contribute back changes. It would have discouraged larger companies with policies against AGPL from doing so, but not everyone.