Honestly, H&P is one of those products that has a solid fan base that will always be there and as someone mentioned, that fan base is not in the U.S. It's akin to soccer cards; no one who isn't into soccer will pick up a set dedicated to soccer prospects.
On a side note, I honestly believe that if ITG made the regular and non-auto/memorabilia cards in the set out-of-the-pack autograph friendly (ie: you could open a pack and hand the card to a player to get it signed with a sharpie without having to worry about the signature smudging, smearing, beading, etc.) the sales would definitely increase.
I know for myself I LOVE getting minor league/junior stuff signed by players, but I HATE having to rub all those cards, etc. Why? No mater what team that player gets drafted/traded to, you don't have to worry about the uniform being out of date. Also, if there was a set I could just buy, sort and then throw into envelopes to get signed by minor league players, that would be awesome. Just something to think about.