Why didn’t Jason, Howard, and Mark get very much lead vocals pre break up?

I get that Gary had the best singing voice and Robbie has that charisma that is hard to ignore but why didn’t Jason, Howard, and Mark get any leads? I can kinda see with Jason because he was mostly brought in as a dancer and his voice most likely didn’t suit the pop material they had (he works best with folky music) but Howard and Mark I just don’t get? Howard has a good voice and I would go so far as to say he has the second best voice in the group. Mark I would instantly assume would get more leads because apparently he was the most popular member back then and he had the most conventionally attractive face but nope. Maybe it’s something else that contributed as to why those three didn’t get any leads and I hope somebody who has more knowledge than me would inform me why they didn’t get any leads pre break up?