Basically: developments in weapons technology, particularly in ammunition, allow for smaller caliber weaponry to deliver rounds with comparable, if not better, penetration profiles and greater force. Additionally, even though the Sherman's cannon was underpowered by contemporary, it could disable any production German tank with a solid hit on the side or rear armor (and even the T-55).

First things first: the Sherman's cannon was woefully insufficient for the rapidly evolving conditions of WWII armored warfare. Barely competent at the outset of American involvement (against 1941/2 Panzer IIIs and IVs), the cannon was quickly made all but worthless in head-on armored warfare by the introduction of the Tiger and the Panther. Worth noting is that the Panther's Kwk 42 L/70 cannon, despite having the same barrel size, could penetrate more than twice the armor of the Sherman's M3 Cannon. I just can't stress enough how poor the Sherman's armament was for the conditions of combat by mid-war; again, though, it could penetrate the side armor of even Tigers, if given the chance to maneuver to take the shot.

Anyway, worth noting is that the T-55 is, with significant modifications, a tank designed during WWII. The Bradley's cannon represents decades of military research in armament, and the velocity and armor-piercing abilities of its rounds are nearly two generations ahead of the T-54/55's original design.

Now, I don't want to give the impression that the Bradley's 25mm rounds were penetrating the front armor plating of T-55s. However, their armor-piercing rounds would certainly have the power to penetrate to the rear and even side armor of the T-55, particularly with the 25mm autocannon's blistering rate of fire and ability to fire APDS (discarding sabot) rounds. To redeem the Sherman's cannon, I believe it, too, could penetrate the rear armor of a T-55.