30K has a number of things 40K doesn't which weigh BIG in its favour:
- Nostalgia factor - all those FW designs based on the original Rogue Trader models? They're the number 1 reason I care about HH.
- Marines - I bloody love marines, and so do a lot of other people. Lest we forget, the reason people complain about marines is largely because they see them so often. The people who complain loudest about marines therefore aren't playing them, and therefore have to be in the minority, or else they wouldn't have anything to complain about, because marines would be Just Another Army. But HH is almost pure marines, and that's got a huge appeal to people like me.
- The Special Snowflake Chapters get their day in the sun - No-one publicly likes Ultramarines; everyone's gotta be different, and HH offers not only an absurd number of new marine Legions, it gives them special rules, unique units, names characters. Iron Hands! Raven Guard! And then, it actually manages to make the Ultramarines cool, by playing up the stuff that makes them different, and putting them in a fight with the Word Bearers, a villain worthy of them. And because the Ultras are only a small part of the Heresy, they don't/can't dominate it.
- Chaos models aren't stupid looking - I mean, this one's mostly me, but I f**king hate the 'Iron Maiden album cover' look that 90% of Chaos Marines have. HH-era Traitors are basically marines but genuinely evil-looking, especially those Word Bearers. They're genuinely horrifying. This is a Good Thing.
- Big stuff - HH is optimised for 3000 point battles which include at least one Lord of War. And let's be fair, everyone loves the big stuff. From superheavies to Primarchs, everyone loves the idea of owning and fielding something world-endingly massive. HH encourages this in a way 40K doesn't, because 30K was designed for it.
- Lack of a tournament scene - Tournament players tend to be the most vocal group about their likes/dislikes, because the issues of balance matter to them more than friendly pick-up players. HH isn't really played in tournaments, because of the tournament phobia of FW, and so the game skews more towards friendly pick-up players who are more inclined to speak positively about their experiences, because they're generally less worried about the game.
- Awesome stuff - the AdMech. Come on, HH is where they started. You telling me we'd ever have had the AdMech in 40K if the HH stuff hadn't sold massively? There's a huge amount of non-Astartes stuff in the HH books, from the varied Knight households to the Solar Auxilia, to three separate and quite distinct AdMech lists.
And what counts against HH?
- Lack of Xenos - yeah, it'd be nice to have Heresy-era Xenos, but really, how different are they going to be to their 40k versions?
- Absurd prices - but GW have cranked theirs up through the roof in recent years, and so FW have become roughly comparable.
- Lack of a tournament scene - but this is a plus for some/many.
So why is HH sacrosant when 40K isn't?
It's basically a very different beast.