The M4s were one of the commonly used tanks during the Korean war which managed to knock out 47 T-34s for the loss of 20 M4s. This was due to the lack of armor the M4 had(92mm of Frontal armor protection) which couldn't withstand a direct hit from the T-34's 85mm gun.
The M4A3E2 Jumbo on the other hand had better armor protection which allowed it to withstand direct hits from Panthers and Tiger Is. So why didn't they use those instead of the standard M4A3E8s?
The primary reason is availability. Despite all of its advantages over a regular M4, the M4A3E2 was built in incredibly limited numbers. Only 254 vehicles were built (including prototypes). Production was never restarted and other assault tank projects (including the ones on the Pershing chassis) died as the war came to an end. There was simply no need for such a specialized tank in the post-WWII army.
The location of the fighting also influenced the vehicles available. It was easiest to deliver tanks that were either already in South Korea or involved in the occupation of Japan. Those were ordinary Shermans, Pershings, and the new Pattons. If shipping a 40-odd ton tank across the world, why take a Sherman rather than a Patton?
Secondly, would the M4A3E2 actually be better than the M4A3E8? The specifics of the Korean theatre of war need to be considered to answer this question. The advantage of the Jumbo was that it could absorb a shot from a hidden tank or anti-tank gun, thus forcing the enemy to reveal their position without any gain. There were no such ambushes in Korea. The main killer of UN armour was mines, not enemy tanks and artillery.
Consider also the terrain. North-Western Europe is chiefly very flat, while Korea is mountainous. The Americans reported high rates of breakdowns as is with 60% of tanks being lost for non-combat reasons. 40% of broken Pershings and Pattons experienced serious issues that required evacuation to the rear, while only 20% of Shermans did. The M32 ARV could also easily recover Shermans, while being too weak to recover the heavier vehicles.
Essentially, by deploying the M4A3E2 instead of the M4A3E8 you would be reducing the tank's reliability, mobility, and firepower (recall that the use of the 76 mm gun instead of the 75 was a field conversion, not an authorized production change) in exchange for additional protection that only applies against one very specific opponent that was not the primary concern in this theatre.
Sources:
https://warspot.net/223-well-fed-sherman
Operations Research Office. The Employment of Armor in Korea. Volume 1
Operations Research Office. US Armor in the Antitank Role. Korea, 1950