It's because it's a lottery, you get what you get because hextech isn't there to give you what YOU want, it's there to make riot money just like any other game's lottery prize where it's usually cheaper to buy than what you're getting or CAN get, but it's randomized.
If people only got skins for champions they owned, it would be WAY too easy to get skins for certain champs that have expensive skins or potentially just get one champ on a new account, get all skins possible on it through hextech, then buy the next champ, get all skins on that one that are possible, then repeat continuously with all champs that you give two craps about and BAM, you now have an account where you can play specific champs with all of their skins when you want.
"Riot you should only give me what I want"
then go buy the skins at the store. It will give you EXACTLY the skin you want, when you want it.
learn2business