Because the Pershing was a heavy tank, and the Sherman was a medium tank. Two different vehicles with two different roles: the Pershing, to support infantry against enemy heavy armour and fortifications, hence the 90mm gun, and the Sherman, to exploit breakthroughs as part of armoured divisions, support infantry in independent tank regiments, battalions and in companies organically attached to infantry divisions, and engage enemy armour when need be. The Sherman could be produced in greater numbers, more cheaply in labour and capital, to supply all of those sundry units, and the US's allies, and even more importantly, could easily fit into the cargo holds of Liberty Ships, LSTs, and whatever might be transporting them across the Atlantic, either to Britain or directly to France and Italy. For logistical concerns and demand, the Sherman was far superior, and as the E2 and (76) modifications demonstrated, more than capable of being up armoured and gunned to match it's opponents, although the E2s were less numerous than 76mm equipped Shermans. Moreover, the Pershing's wide body and tracks, combined with it's weight, subjected it to the same limitations as the Tiger and Josef Stalin tanks in terms of operational and strategic mobility; when a crossing over the Rhine was gained at Remagen, it proved very difficult to transport the Pershings across, while Sherman tanks could manage via the bridge and, later, the pontoon bridges.

I believe Steven Zaloga has covered these controversies in his writings on WWII armoured fighting vehicles, and The Chieftain's Hatch has also done written and video examinations of the issue(s) as well. If anyone cares to add, by all means, do so.

In addition to what /u/DuxBelisarius said, I'd like to point out a few things. First, tank-on-tank fighting was fairly rare, particularly on the western front. Most rounds fired by tankers were High Explosive (HE) in support of infantry or to take out anti-tank guns. In this, the 75mm gun on the Sherman was superior to both the 76mm and the 90mm, both of which were high-velocity guns. Second, US doctrine at the time was that enemy armor was to be engaged with anti-tank guns or tank destroyers. The Germans also tended to use anti-tank guns and jagdpanzers against enemy tanks. To quote GEN Leslie McNair, chief of Army Ground Forces in 1943, "There can be no basis for the T26 tank other than the conception of a tank versus tank duel—which is believed unsound and unnecessary." And, "Any attempt to armor and gun tanks so as to outmatch antitank guns is foredoomed to failure... There is no indication that the 76 mm antitank gun is inadequate against the German Mark VI (Tiger) tank." In other words, his opinion was there was no reason to get into an armor race, because tanks would always be overmatched by anti-tank guns, and the current 76mm (and 3") AT guns were able to deal with the Tiger I.

The deficiencies of the 75mm Sherman in tank-on-tank engagements was not obvious until after the Normandy invasion. In North Africa, the 75mm was quite adequate to defeat the PzIV tank. Army intelligence (correctly) assessed that the Tiger I would be produced in small numbers, and the 76mm/3" AT guns were adequate to take them out. Intel from the Soviets on the Panther was slow to reach the western allies. Therefore, there was little impetus to continue development of the T26, and certainly nothing to warrant slowing M4 production to ramp up for the T26/M26.

Remember also that, do to one of the most colossal blunders of the war, the Allies captured Antwerp, but failed to push on and secure the Scheldt Estuary, meaning that until the Scheldt was secured and cleared of mines at the end of November 1944, the Allies were dependent on the single surviving Mulberry at Normandy, eventually supplemented by the heavily damaged port at Cherbourg, to offload tanks.