There are. The Democratic Socialists of America has grown a lot over the past few years. They even managed to get four member elected Congress this last election cycle.
It seems as if it should have happened "naturally" one or two generations ago.
I have no evidence, but I suspect McCarthyism and the following hard crackdown on leftists that followed during the cold war greatly slowed down this "natural" growth. It took a generation removed from the cold war for this growth to start up again.