They did not know in fall what the hygiene regulations in December would be, and since organizing the congress means lots of effort and expenses, they decided to wait it out.
I'm from Denmark and have been to conferences in Germany in 2022 and 2023. Never heard about COVID regulations still in effect.
In fall 2022 no one could be sure there wouldn't be regulations in effect by the time CCC took place. Even though it was probably considered unlikely by most, they couldn't or wouldn't take their chances.