Seriously why does it cost money? Cause the only reason I can think of is so that they can squeeze as much money as possible out of their players. I wouldn't mind so much if it wasn't so god damn expensive? $20?! Really?!
I highly doubt it's a technical issue since Blizzard allowed all of your items transfer servers for free in Overwatch.
It's because riot provides us with a free game. They need to make money somehow.
They don't want people transferring servers willy nilly. From what I've seen through the Riot API, it looks like server transfers have a big potential to mess up your match history (which might be part of why you have to re-do placements after transferring). A $20 one way price is too steep for the person who only wants to play a game or two then switch back, but is fine for someone who's home region will be changed for the next year or more.
