Celebration is also of little interest to gamers, but a thing for star wars fans. Personally, as a not-so-much star wars fanatic, I never even heard of it. Whereas GamesCom is one of the major gaming events and will offer SWTOR great publicity to gamers worldwide.
It's not hard to understand why they'd pick GamesCom.
But considering how they expected this game would draw more of it's players from Star Wars fandom and not the general gaming community, it is probably not the choice they would have made 7 months ago.