Because windows phone is not supported?
all it is, is playing in the browser. Windows 10 Mobile is supported, but it blocks you on 8.1. It does not make much sense, is there any reason behind it? Is the browser lacking some sort of support....other frameworks like Air Console work no problem with 8.1, even using web sockets. So I don't see on the front-end why it would not work.
It is a big deal in my house atleast, almost everyone uses a 8.1 phone. When people come over to play these types of games, they usually use my spare 8.1 developer devices when their phone is dead, or they forget a phone.