It's probably because it's essentially a book for Cap fans to buy if they want even more Cap on top of the regular Cap monthly AND All-Winners, which has their other favorite heroes in addition to just as much Cap as USA has. That shouldn't amount to extremely high sales per issue.

Out yonder, under the shining vault, among men the saying goes: 'Man, be thyself!'
At home here with us, 'mid the tribe of the trolls, the saying goes: 'Troll, to thyself be-enough!'