Sherman could have been designed as a more effective gun platform from the outset, but powerful interests in the military, procurement, and industrial sectors lobbied for the mass-produced, low-velocity 75mm...because it could lay down a large volume of fire against its targets.
It was never meant to be a main battle tank.
The British Army nearly missed out on getting the 17 pd., high-velocity gun for the very same reasons. The only reason they did was because those in favor of the low-velocity 75 were outmaneouvered in committee by a few desperate men who knew the truth about the type of enemy their tanks would be facing.
In addition, I believe that the overall dimensions of the Sherman were dictated by the problems inherent in transporting a tank across the Atlantic on the types of ships available at that time. A smaller, narrower tank would take up less space, and more of them could be shipped at one time.