I'll hazard a guess and say it's probably since Halloween and Christmas are slightly more universal than the US Thanksgiving holiday. If they do something for Christmas, it'd be nice. Beats me why you'd be upset if they chose to give us something for the festive season, but then again I would've thought the answer to your first question was obvious...the US is not the only country playing the game

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Even those with thanksgiving holidays of their own, why should they celebrate it when the US does? Then again, as an alternative, Anet would have to do something for everyone's own little national holiday...and that's alot of events to play to.
Online game, universal appeal between countries and nationalities. A nice melting pot of players ... so why would the rest of us wanna observe a holiday only relevant to one country in many?