That's all speculation.  It's a mystery because none of us here have produced any documentation on the decision.  We can reason why they didn't but we don't know.

I was watching the "Operation Think Tank" video on The Chieftain's YouTube channel and got an insight.  My current impression is that with the short life of individual tanks and the speed of development in WWII that it did not occur to the planners that M4 Sherman 76mm tanks would be around very long.  It probably did not seem to be worth the trouble to develop a new HE shell and associated manufacturing line.  Besides, the M4 model carrying a 105mm howitzer was accepted into service at virtually the same time and it would be a much better HE thrower.    The new M26 Pershing tanks with their 90mm guns would soon be available and be better hole-punchers.   

According to this presenter, there were some plans in late 1945 for postwar units to have mainly Sherman M4's fitted with 105mm howitzers to dole out HE and a much smaller percentage of M26 Pershings to deal with other tanks.

https://youtu.be/C1sN52CBKi4?list=PLEAEU2gs2Nz-aSi3PpjNI9Q4klDGi421D&t=1199