User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-25259914-20141016164400/@comment-21371438-20141016211527

i'm gonna try to answer

apparently, if you mean "why do variation appears? because they were too special to get a subspecies" is kinda... true

aside from Deviljho and Rajang, who are... something different (what people nickname "powerhouse", hinting those two hyper-powerful monsters who once had the "variation pattern" as signature of "some high-level-of-power creatures")

let's just imagine a Brachy Sub or a Gore Sub

Brachy sub would need a different kind of slime to be stronger than the original, but there is kind of nothing GOOd enough to be "over" original brachy, since its power is the strenght applied to the explosive status (and istant explosions when enraged... doubt something would get greater)

on another hand, Gore Magala has an unique story pattern, from Gore to Shagaru, would be quite impossible to get a sub or a Rare without overpowering the already-existing complete evolutionary stadium (so there goes the rare...), also, its element is like Brachydios: hardly enough they can create what they did with Zinogre (different element sub), so... Variant was the best guess they could make for such monsters

other monsters have lately got a variant (Red Fatalis, Kushala Daora -reworked-), but i think you meant the "mascotte"/"flagship" monsters, who usually followed the sub + rare pattern -''Rathian excluded... rathian is tied to Rathalos''-)

i hope i answered.

yes, i wrote Goo while talking of Slimes... i know.