Hextech crafting is pretty heavily monetized.
There's several artificial limitations placed on the system, making it more complicated and less fun, in order to goad suckers into paying Riot more money.
Hextech crafting pushes you to buy more champions. You can only get one hextech box from one champion per year. Meaning anyone who wants a chance to get those chests is going to need to own 50+ champions. Mind you, I've played this game for years and I only have about 30 champions. Oh, and don't forget you can't earn chests from champions you do not own, even if you do get an S. So using free week champions isn't possible.
Key fragments are limited so players will feel obliged to pay RP for keys to open those chests they have sitting around doing nothing. Every key fragment in a given month becomes increasingly unlikely to obtain. Not only is it it difficult to get 16 fragments in a month, but anybody who slacks off for a month will find it even harder to ever catch back up later.
And on a more minor note, of course there's the admitted favoritism towards premades to push people towards Riot's ideal of filling out the game with mostly premades and treating solo queuers as nothing but filler, as well as the weekly/monthly reset on chests and key fragments to keep people addicted to grinding for them.
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What's funny, is if there was one part of the Hextech crafting system I had to praise, it would be Hextech Annie, which I've seen multiple complaints about on these forums. Hey, the fact you need 10 different gems at 4% chance actually reduces the variance in boxes you need than if they had decided "lol let's throw it in at a .4% chance". It's a perfect way to take advantage of big fish who want to throw money at the game, without bothering other players.
There should be rare skins, it spices things up. I've seen some interesting conversations when Hextech Annie shows up in a game, which is really saying something, considering how little real conversation there is between randoms in league nowadays.