I thought about that also. My conclusion: 1) Not political (or really controversial at all), 2) Meme is newish so not over done yet, 3) who doesn't like star wars? 4) Everyone on the sub knew right away what they wanted to post, so there was no lag time between designing a "logo" and rallying the troops.
I get the fun of /r/place. I get the flags. I get the characters. I don't get the big block of text from Revenge of the Sith, and (mainly) how supporters of that beat out other communities.
I'm still impressed all things considered.