The existing market is there making money and all they care about is the money. In comes Mozilla with a superior OS, because lets face it besides the abundance of apps Android is a rotten locked down turd with buggy slow core, but all the cellphone makers are already making huge dollars off android so not a single vendor would install this on their phone or give it a chance.
It all comes down to a decision the cellphone makers had: install this OS or this one.
It seems Google is even offering makers money, bonuses and ad revenue to use their OS so its a definite no. No one is going to touch a third party app with that kind of deal in place with Google.
It doesnt make it right. It is a flaw in capitalism that promotes monopolies and bad crappy products like Android.