There were a number of other attempts by companies other than DC to revive the superhero genre in the early mid-50s (Fighting American, Bullseye, Captain Flash, Strong Man, The Avenger, Blue Beetle, and others), and they also fell flat.
And The Flash, beginning in Showcase in 1956, met with indifferent results at first. It wasn't until 1958 that things began to take off for the DC revival.
1955 just wasn't the right time yet.
I'd attribute this to two factors: first, by the late '50s, the Wertham hysteria had died down, Comics still had a fairly bad name, but parents had other things to be concerned about--which leads into the next point, which is that the leading edge of the babyboomer generation was just hitting 12 in 1958, and was eager for comics that were a bit more sophisticated than what had previously been on offer.
I was one of those kids. I'd been completely unaware of the existence of any of the earlier characters I listed above, not having been of an age to determine my own purchases very effectively, or to shop on my own. By '58, we were beginning to be a market force independent of our parents.

And one other point: DC had distribution guaranteed. As the sister company of Independent News, DC could be confident of a place on any rack in the US that had any comics at all (except for those that sold Classics Illustrated, which was a completely different sort of animal). None of the other companies except Dell, which eschewed superheroics entirely until the early '60s when it was fighting for survival after the divorce from Western Publishing, could say that.
Who am us, anyway?
--The Firesign Theater