I've always felt that the main reason people hack FireRed over Ruby and Emerald is that the ROM is far better understood.
What I mean by this is that the various functions in FR are well documented and there are several hacks for FR to deal with FR's most obvious weaknesses.
The other biggy is probably that folk like myself and Darthatron personally hack FireRed, and throughout our time hacking it, we have produced several useful pieces of ASM which we have shared with the community.
It leads to a position where, if you want the "best features", you kinda have to hack FireRed. (Note, I'm not saying FR is better to hack than Emerald, but a lot of people seem to think that it is...)
Finally, there's the well documented (but tremendously poorly explained) corruption issues with Emerald.
In fact, in my opinion, Emerald is the best ROM hack base given its base features, and yet, I still personally prefer FR!