The cowmangler doesn't "break" the game like some of the other banned unlocks, but the outcry has always been the charge shot at it's core, with the ammo as the cherry on top (your demo has many more pipes to spam if your pocket is running it, as well).
It's one of those bans that gets put in because, while sometimes you can hear the shot, taking a mini-crit charge shot across the map, from a spot you can't hear it from, the second you decide to peek through a door to get info, feels like the most random, unskilled, and unenjoyable thing. Burning your clip hardly means anything at certain distances; you have plenty of time to reload and the risk is worth the reward since your 140dmg laser beam can hard stall/kill pushes instantly.
A lot of weapon ban arguments come down to the philosophy. Do you allow it because it doesn't disrupt the flow of 6s past a certain threshold? Or do you disallow it because the mechanic of an unskilled cross map death laser isn't something people want to play with/against?
For reference, The Cow Mangler 5000 was allowed once from ESEA 22-26, and then a second time during ESEA 29-30; addition due to the first philosophy, removal due to the latter.