From the article:
The winter battles of late 1944 and early 1945 finally demonstrated to the US Army's high command that the M4 Sherman, though providing excellent service in previous years, was wholly inadequate to confront German heavy armour and anti-tank guns.
Furthermore, it fatally undermined the prevailing US tank doctrine, which had prescribed leaving the engagement of enemy tanks to the Tank Destroyer battalions, freeing the tanks to support the infantry and exploit gaps in the enemy lines. A new armoured vehicle was needed to better suited to the realities of modern warfare. Fortunately, after a good deal of political debate, the T26E3 heavy tank was put into production in November 1944.