The Sherman had to be small enough to fit on standard rail cars and pass thru standard rail tunnels (particularly in the UK). It also had to be light enough to be loaded and unloaded alongside a liberty ship using only on board cranes and equipment (not many full on harbours on little islands in the Pacific).

It also had to be light enough to cross the mobile engineer (Bailey etc.) bridges in service at the time. Early in the Pershing’s service during Market Garden, they had to be left behind because they were too big for the engineer bridge, so only Shermans ended up crossing and doing the fighting.

Few shooters [combat arms] as compared to non-shooters in the U.S Army. This is why. Logistics. Getting heavy gear to the scene of the fighting and then maintaining and sustaining for a long period requires that logistic train. Korea is on other side of the world almost.