So, I'm curious. What's wrong with erotica and "manchest" covers?
I don't think the OP's intent is to say that there's anything wrong with either. As Joe suggests, though, they don't appeal to all audiences. Giveaway hosts on BookFunnel and Prolific Works both often indicate how they want to focus a particular giveaway.
It's worth noting that it isn't always erotica and man-chest covers that are excluded. I was looking at a fantasy giveaway not so long ago that excluded something like "all fantasy based on living religions, particularly works that read like Bible fanfics." When it comes down to it, most organizers are creating giveaways that will draw in potential readers (and email subscribers) for the kind of material they write themselves. Needless to say, I've also seen erotica-only giveaways for the same reason.
From my point of view, it's much better for an organizer to give people submitting an idea of what the focus is. I was once part of a Halloween horror/fantasy promo in which I used a Halloween-setting YA fantasy. Imagine my surprise when I visited the giveaway page and discovered my YA surrounded by scantily clad witches, vampires (undead manchest) and werewolves (partially human manchest). In other words, the heat level wasn't made clear in the promo application. I guess people who knew the promoter knew what the emphasis was. My book was the only one that didn't fit. I had a good laugh over it, but my misplaced book didn't do anything in that promo. If it had been labeled erotica or at least steamy, I would have known better how to approach it.