In addition to what other have said about extensions, ad blocking, no telemetry, and some nice features like tab management, it works in the Apple ecosystem. If you use a mac and an iphone, you get the native sync services for history, bookmarks, passwords, etc. Makes sense that they're focusing on that.

Developing internally initially, as opposed to making it open source, was probably the only practical way for a small team with a specific goal to get there. I don't see any problem turning it over to community development for secondary platforms. If it gets adoption, great. If it fizzles out on linux and windows, nothing is lost.