Well, he is! His cello concerto is standard repertoire across the world, and his New World Symphony is standard in the US. He should also be known for originating the ET theme [1,2], but that's not how things work.

[1] https://www.youtube.com/watch?v=vDPkMvUAgqo&t=1955s

[2] https://www.youtube.com/watch?v=2-qrMz-JAzo&list=RD2-qrMz-JA...