But what about humans? Yes, it's a bit boring, but they have very powerful troops.

It seems to me that the way the question is posed in the topic implies that it's not about discussing tactical and strategic possibilities, but rather about appearance, composition, and so on. That's why people don't vote for humans; their originality ends with the city architecture (although even that can be compared to Lord of the Rings, for example). And demons are simply not liked; it's unlikely that it's because they are unsuccessful; they are simply presented as the main evil, and at the same time, an image of losers is consistently created – there are many of them, and they seem dangerous, but they always lose with disgrace. In fact, their city and creatures are very, very good in my opinion.