U.S. Army armored doctrine and equipment was wedded to the idea that tanks didn't fight tanks. Tanks were to be used for exploitation/pursuit following a breakthrough while tank destroyers were supposed to fight enemy tanks. Hence the lightly armored Sherman with it's low velocity 75mm gun that fired HE and solid shot projectiles. The powers that be, including Patton, persisted in this view even after tanks encountered German tanks, incl the first Tigers, in N. Africa.
The Brits got the message and scrambled to adapt the high velocity 17pdr anti-tank gun to the Sherman in time for Normandy. This gun was offered to the US Army who declined it based on their mis-guided doctrine and the "not invented here" syndrome. The US was also committed to volume production of the Sherman in spite of it's gun and armor inadequacies. It was, however, a good automotive design which was reliable and easy to maintain. The US quickly fitted a higher velocity 76mm gun to the Sherman in 1944, but it never was as effective as the 17pdr.
Funny how army doctrine works. Take now for example: "Hey, lets use .223 ammo so we can carry twice as much". Oops, we're supposed to score THREE hits to ensure a kill now. That's good math.