Genuine question about Captian America

What caused him to not be as popular as the X-Men, FF, and Spiderman. Because I look at him and the time period he was created in and it genuinely surprises me. Captian America someone who is meant to embodies the idea of America and the Freedoms and Liberties it grants you'd think would be amoung the top heros but he isn't. Espeically since he came out during the height of world war 2 and was still around during the cold war.

Did it have something to do with the Vietnam war? or was he declining in popularity before then and the war further speed things up?