Riot's reasoning for not making a sandbox mode:
"We don't want dedicated players to be better than people who play once a week because it would be unfair."
Basically it. They've basically ignored the idea of Elo and thought of it like "well a good player would be with bronzies because of sandbox" rather than the way they should think, which is "well a good player would climb out of bronze because of his training in sandbox."
Like wtf rito? You want us to suck?