Well, whether the graphics are really that great is debatable. For example, in "Civilization IV," the graphics are, in my opinion, better.
Oh! There are also fans of Civilization here!!! Awesome, I love it too (but I think the third one is better).
Maybe I'm a conservative... But the graphics in the fifth Heroes game are actually pretty good. Maybe not the best, of course. This is actually its main qualitative difference from the third game. But I also like the graphics in the fourth game. In general, I've noticed that I prefer a compromise between 2D and 3D.
Here's another real-life example to confirm what I said earlier. I played a strategy game (Stronghold). I was completely obsessed with it, and I loved everything about it. Then Stronghold II came out with 3D graphics. But with it came different feelings. I can't play it with the same enthusiasm as the first one. The graphics seem great, and everything is well-drawn.
But I also noticed that there are a lot of bugs and unfinished elements. It's generally poorly designed.
Therefore, I've concluded that if developers focus all their attention on the graphics, the game will probably not be one of the best.