Jungler role has too much influence on the match balance.
He can influence the whole game by feeding his lanes.
If a sololane (or bot) is behind, the match is not necessarely over because the other lanes can still be a win condition if doing well.
If the jungler is ahead, he just win alone the other lanes by giving free kills\assists to his team and letting snowball them.
It's totally unbalanced to give such power to a single role in a teamgame, it's like a giant RNG who determine your win\lose condition