Just as there are different types of pants, there are different types of kilts. There are jeans, khakis, fatigue (aka Camo), dress, Tux Pants, and ripped jeans. Are these all pants? Yes. Are they all worn by the same people? No (or at least, not necessarily).

Likewise, there are different types of kilts... UK's, FK's, RK's are like "Jeans" (on the whole) or Fatigues. Union Kilts and 21st Century and USAK Solid 5 yarders both do the matching vest thing (like a suit). LOTS of places do the "full 8 yarder" thing (like a tux). SK, SWK, USAK do the Casual thing (like Khakis). UK, R Kilts (leather ones) and TRIPP do the "punk rock" thing in leather ones and cheap knock around tartan ones.

Are all the items in the first part of this post pants? Yes.
Are all the things in the 2nd paragraph kilts? Yes. are they each different from one another? Yes. SHOULD they be different from one another? You decide, but I will say this... wouldn't it be boring if they were all the same thing? How would any one company make something better if they were all the same thing? Things that are stagnant will die. Things must evolve to survive.

They all have their place. If they ALL succeed, then you'll have people being able to distinguish between the different KINDS of kilts like you have people differentiating between different types of Pants. Does anyone confuse Levis Jeans with Tux pants (except at "country western weddings"... J/K)?