Main reason: you can play it on Linux !
I'm also really interested in the Open Source nature of the game making it easy to try new ideas, balance changes ...
Also I love FiveAces streaming :-)
Why do you play OpenRA?
For the most part I missed out on the first generation c&c games; only experiencing them through awkward and incomplete pirated versions and the PS1 games. Being that I wasn't conscious of online multiplayer as a thing until mid-2002 either I certainly missed out on their online scenes as well. So for me it was getting to visit these classic titles with a revitalised online scene but also with modernised interfaces, net code and balance changes.