CCC often reveals new software vulnerabilities. I don't know if it's deliberate, but if they reveal those vulnerabilities at Christmas, they ruin the holiday of whoever is responsible for fixing them - quite likely someone working for a big corporation.
People associated with the CCC most often do responsible disclosure. Therefore you can expect vulnerabilities revealed during C3 to be already known in advance to the vendor. AFAIK an exception to that are jail breaks, which have quite a history of getting announced during C3 without disclosing them to the vendor first.
Or "ruining someone's holiday" is a secondary effect. Simplest explanation is that it's easy to get vacation time from your "normal" job at that time compared to other times, so if you want to make sure as many people can make it as possible, those dates are likely to be the best.
Where were you trying to travel from? Last time I was at the congress, most people I talked to arrived by train, myself included, even if it took a day of traveling.
