The multi-year Google vs Oracle lawsuit put the likes of Amazon and Facebook to unease. And meanwhile there was this safe and performant language with no strings attached under a nice Apache license. They quickly made a Foundation around it as a 501(c)(6) - trade association, and not a "public good"-kind of a non-profit. This essentially means that as long as you're a member and pay the fees you won't be sued - exactly what all these companies wanted. So now they all keep up the funding and employ some of the compiler developers to keep the lights on for next 20 years or longer.

The fact that the language itself is really good for what they need it for is obviously the major reason why they support it, but the legal side of things definitely helped with adoption.