Generally, if a Pokemon has access to Thunder or Blizzard, it also has access to Thunderbolt or Ice Beam, and can readily use that option (for about a 20% damage discount) if it cares about reliability. Meanwhile, Focus Blast sits at the intersection of providing what's often pretty synergistic type coverage (for instance, letting Alakazam fight back against Steel and Dark), and also, for a lot of Pokemon that appreciate having that particular coverage, being the only move they get that takes advantage of it (at least if you're not willing to give up a 50% power discount to go to Hidden Power, which completely cancels out the effect of being super-effective to begin with...not to mention some Pokemon like Tapu Lele don't even have that option at all).
So other than the few special attackers that can claim access to Aura Sphere, such as Mewtwo or Magearna, Fighting-type coverage usually puts them in an awkward spot where it's this notoriously unreliable attack, or nothing, and they use it only begrudgingly.
