Elementals were, and really still are, kinda a novelty. Giant genetically engineered humans. I'm not surprised the writers have chosen over the years to make a few prominent personalities Elementals. Just the word itself brings up this image of a towering strong man or woman, rather than having to say , "Khan Joe is enormously tall and strong".
He's an elemental...fewer words required, mental image achieved.
The aero pilot phenotype, I find, hasn't been that popular with many BT fans, but Elementals are. Not to mention battle armor in general has been popular with BT fans.
Early on, in things like the Clan Wolf Sourcebook, elementals were treated almost like second-class. A personality profile on a Star Captain noted that it was unusual to find an Elemental Star Captain and in that book and the Jade Falcon Sourcebook, Elementals of high rank were extremely rare. I feel like the writers jumped on the Elemental fan train over time.