From: https://en.wikipedia.org/wiki/StG_44

1. It was poorly rated: "The British were critical of the weapon, saying that the receiver could be bent and the bolt locked up by the mere act of knocking a leaning rifle onto a hard floor.[15] A late-war U.S. assessment derided the StG-44 as "mediocre", "bulky" and "unhandy", declaring it incapable of sustained automatic fire and prone to jamming, though the report accepted that its accuracy was "excellent for a weapon of its type".[16]"
2. The US already had the .30 caliber M1 carbine that they converted to the M2 carbine: "In 1944 the US added an automatic fire capability to the M1 carbine, and issued it as the M2 carbine with 30 round magazines, fulfilling much the same function. Kits were distributed to convert M1 carbines to M2s."

https://en.wikipedia.org/wiki/M1_carbine#/media/File:US_M2_Carbine.jpg