The reason few fire types are successful defensively is not because of their type, but because of their stats since most of them are geared towards offense.
Many ice types on the other hand have excellent defensive stats and support movepools but can't be used to their full potential because they don't have enough resistances.
Fire already had five extremely useful resistances and among the types that are weak to Stealth Rock it was unquestionably the best one defensively.
Its worst weakness, water, was a problem in BW because of perma-weather but that's gone now too.
The WoW buff further improved fire types defensively as they can absorb that move, which is everywhere now.
So yes, giving fire an extra resistance compared to types that needed it more (ice, grass, bug and psychic come to mind) was an extremely poor choice from a competitive standpoint and on top of that it makes no sense flavor-wise.
A common joke is that the only reason fire resists fairy was to make Mega Charizard X and Mega Blaziken, the two most marketed megas, more appealing. I'm starting to believe it's more than just a joke.