The main thing is that Mozilla does not have an free and open RIL to use.
Also Firefox OS is a partnership with hardware vendors and folks such as ZTE, LG, TCL don't see value in opening the bootloader. The baseband providers don't see value in opening the RIL... and so it goes.
Where it comes to Android, there is already Hayes-RIL for AT based basebands.
There are of course also freesmartphone.org's fsogsmd and oFono. You could probably develop an Android layer on top of them if you wanted, bringing great value not just to Firefox OS, but also to AOSP itself.
Also Firefox OS is a partnership with hardware vendors and folks such as ZTE, LG, TCL don't see value in opening the bootloader. The baseband providers don't see value in opening the RIL... and so it goes.