I just thought it is a strange name and why it isn't called anything else in other games...
Thanks.
It's called Indigo Pleatu because the pokemon league in HG/SS is set in Kanto. Because Kanto was the first region the pokemon league there has a name: Indigo Pleatu
First: It's Indigo Plateau. Second: Plateau is some kind of plain on hill Third: If you are asking why Indigo, then it may mean something like colorings, because there is a lot of flowers
Well, the other ones do have names, they just aren't stated, hoenn has ever grande city, kanto and johto have the indigo plateau, and sinnoh and unova (at least for now in unovas case) have un-named pokemon leagues. why some are named and some aren't I don't know, but indigo plateau makes sense, on an inclined plane (Plateau) and the flowers are the color "Indigo" or at least many of them are. hence the name Indigo Plateau. Ever Grande City is the Largest place in the hoenn region, hence the name Ever Grande (which means something like... Forever Great City.)