So, why did Harvey's Thriller line fail so quickly?
I don't want to judge too much without actually having read most of them, but one factor does stand out:
Lord Lazee, the world's laziest villain!!
Oh, don't get me wrong, I love Lord Lazee--if I were to be a super-villain, Lazee would be the way I would go! But...
He's back again in Jack Q. Frost's second cover, just as Brainstorm returned for the second story of Pirana after menacing the hero in his debut.
That's just speaks of a lack of creative imagination--not only are most of your hero's knock-offs of existing Marvel/DC books, but for most of those books you can't come with with two issues worth of villains!!
Given the flow of genius from the Marvel Bullpen, and the sheer imaginative nuttiness of DC's Silver Age output, Harvey's super-hero output did little with their concepts or covers to grab the buyers' eye.
Of considerably more interest was the back-up feature in both issues, Miracles Inc. Their first appearance, in Unearthly Spectaculars #2, was by Wally Wood, and he treated this odd collection of heroes as a bit of satire, an Inferior Five-style take on a Doom/Patrol/Metal Men type group. That slant was lost next issue, however, as Otto Binder and Joe Orlando went more straight super-hero. However, their tale, "Rent-A-Hero," did predate Heroes For Hire by a number of years...