One reason is that there are no women in it. I don't mean it in a gay way, just that there's no pointless romantic distraction, no fucking grrl-powah scenes, just male camaraderie. It's also an interesting look at leadership styles and the importance of discipline.
The greatest part is that it feels very much of that time. There are no stupid "war is, like, bad man" scene thrown in for modern audience's benefit, no black guy for diversity, no shoehorned "look how racist they are!" content.
I think another great part of the movie is that they don't touch too heavily on politics. There's not much talk about rights or wrongs of the French Revolution, the nature of monarchy etc.. It's just two groups of soldiers doing what soldiers do, and showing respect to each other.
