I am not much of a maths player so I can't tell you why mathematically, only that shoving 72o at 20bb is a losing play. I think everyone here would agree. Someone else can probably explain why. To me it is just obviously bad. Maybe you will get a fold but you're essentially risking all your chips to win a very small amount. If I see you shoving really light I can just wait for a decent hand and call you and win since if we are 20bbs deep I don't need to get my stack in super light yet. Shoving 72o isn't even on the nash charts until you are at 1.6bbs but at that point you're shoving almost any two (except 23o - 25o which you shove at 1.4bb).