Well you have stuff like the steam client saving stats, having item support, having marketplace for buying it and selling the items to pay for it, that kind of stuff. For LoL to add a compendium, they'd need to open a specific website that can support the load to buy it(which obviously isn't possible so it'd be down for like 3-4days), then if you wanted predictions and shit you'd need a way to track the stats of the players, teams and whatever so they'd need a bunch of interns updating all that shit since it's not done automatically like in dota, and they'd need a new interface to track all your shit. I guess they could do a compendium lite or something, where you just buy some cosmetics ingame via the normal store and it just gives you a skin that is updated based on results(but not dynamically, more like every week they updated the client with the new skin).