HOMM1 had a more symbolic aesthetic which I happen to preffer to 2 (and all the other games in the series). Also, it had smaller, tighter battlefields. After so many years, replaying HOMM II after replaying HOMM I had me feeling like II was just bloated in too many directions and had units swimming in the overly large battlefield. HOMM III also felt really bloated afterwards. I ended up having the most appreciation for 1 of the early installments. I tas it's flaws, sure, but it's a better game than either II or III.
Also, inovation died and overly conservative fanbase won when they couldn't handle Heroes IV. HOMM 1-3 are really simple games, there's more complex indies on Steam out there, and II and III can kind of be seen as decline on the tightness of HOMM 1. Heroes III drew a humongous, enromous crowd of people who are, when you get down to it, really casual. It's probably the most played game when it comes to girls from a time no other game was really marketed to girls, so for many girls it was really the only game they played. What III had was graphics which made it look, to the casual crowd, as a "seirous" game, what with the "realism" and all, tacked onto a game which was really, really simple. This is what makes Heroes III so much more popular with huge seas of people to this day - it makes them feel like they're good at a "serious" game, while secretly being a casual phone game with a specific kind of graphics. No joke. I played it a lot, so did literally everyone around me, but I calls them like I sees them.
Add simultanous retaliation to it, so that it's not just a game of who strikes first, and the suddenly added depth simply chases most of the fanbase off. More people were put off by sensible development of the system than there were people put off by by the actual screwups of Heroes IV. Heroes IV tried to innovate and this was met with so much backlash that noone really dared develop the game in other ways than graphics, more or less, ever. So it kinda stayed a phone game with ever more, well, "realistic" graphics, so to speak. Heroes V was painful to play, the graphics got more boring, the specc demands for what's basically a phone game went through the roof, and the gameplay went backwards, so if you got hooked on IV it felt like you had something aimed at kids in your hands and it just wasn't as stimulating as it was when you were a kid.
Kings Bounty didn't innovate in that particular direction but I liked it a lot better than anything after IV, because it at least shook things up properly. Or rather, it went back to King's Bounty, also a game I played a lot, which was kind of deeper and more interesting than HoMM games. HoMM was more like a really simplified deathmatch / multiplayer for KB, and that's not really all that KB was.