Android's new floor is 512MB of RAM and that ain't happening in a $25 phone. Firefox OS will run on 128MB, fitting into an entire category of phones that modern versions of Android simply cannot.
B2G is different than Desktop Firefox in some ways, for instance it uses multiple processes (this is being worked on for Desktop, it's already in use for plugins for instance) and supports some new APIs to access the hardware on the phone that you'd expect: https://wiki.mozilla.org/WebAPI