HoMM 1-3 were fairly straightforward iterations on the formula. Most of the changes in the second and third games was just adding more stuff and filling in some obvious holes in the previous game. HoMM 3 was the point where the game was sufficiently complete that doing this another time was no longer a viable strategy, and HoMM 4 had to figure out how to make itself stand out as a new game. They didn't really succeed at that, and while it's a good game it's not really a better game than 3. 360 running out of money while developing HoMM 4 was unsurprisingly a big problem for the game.

HoMM 5 was a much more successful rethinking of the formula, and while the game as originally shipped had some major problems, with the expansion packs it's arguably the best game in the series. It's a meaningfully different game from 3, though. Many of the changes made the game different moreso than better or worse, so some people prefer 3 and some prefer 5.

I never played 6, but what I heard was that it had a similar problem to some of the Civilization games where HoMM 5 plus expansion packs was just a much more complete game than 6 with no expansion packs, and then it didn't sell well enough to justify continuing development to "finish" it.

Fun and balanced castles, expansions and mods, playable and bug-free on modern systems. It's a great game, or actually several games in one: town-building, map exploration, RPG, chess-like strategy battles

I think 3 was the series' peak. IIRC 4+ didn't add much, if anything at all, apart from superficial improvements, and there were more good game options out there too.

Also: nostalgia. There are many who were gaming during those peak times who spent a lot of time on these games.

never played 4 or 5, but from reviews at the time it sounded like 4 and 5 were just lower effort games coasting on the reputation of 2 and 3