I think its due to the simplicity and the way they seem to make it fun. I have not come across one title that really turned my crank, that is until now. Age of Kings takes place in just that, the age of kings and part of what makes the game so enjoyable and involving is that the single player campaigns are all derived from actual events that took place during that period of human history when kings ruled. Campaigns that revolve around William Wallace, Joan of Arc, Genghis Khan, Barbarossa, and Saladin will test all of your strategic skills as you strive onward to victory. You can be a part of history and decide the outcome.
Parents take note.
Probably one of my biggest pet peeves I have about RTS games is that the characters and the objects such as buildings and vehicles are out of scale. Too my great satisfaction one of the first things I noticed about Age of Kings is that everything is perfectly to scale, even the castles and monolithic buildings are scaled proportionately. The graphics are absolutely brilliant, this is without a doubt where this game shines the most. The terrain is remarkably realistic, unlike other games in the genre the terrain never seems monotonous, every rock, pond, tree and bush all seem to be completely and naturally random. The artists of this game did a fantastic job of recreating buildings for there periods.