They never knew what to do with him.
His original concept was of the long-lost brother of Cyclops who's only mutant power was to randomly explode from time to time unless he wears that special power-regulating black suit.
He wasn't meant to be the second Summers superhero, he was supposed to be the civilian mutant that's occasionally caught up in as a hostage or manipulated puppet of some evil mastermind.
( he had that in common with Polaris ...so ... )
Then Claremont came along and decided that Havok should be the brother inferiority complex X-Man.
And after that they were just putting him in books because he was a known character.
X-Factor in the '90s was just a holding pattern for otherwise forgotten characters.
The Madrox detective agency era was meta about that basic concept, and so much more fun to read, but it was a holding pattern book too.
And lets not forget the time Havok got shuttled off into his own book far and away from anything relevant to the rest of X-comics.
Or the other time that exact same thing happened.
Or that time they handed him off to an Avengers book and apparently told the writer; "do whatever you want to his pretty face."