It's traditionally and still a very DACH dominated event, for better or worse. Travel by regional/interregional trains around this time is perfectly fine, and it's a time where it's quite easy for a whole big community to synchronize for taking vacation.

Remember that this is not the type of Congress where people go on paid time. There are no sponsorships and generally no commercial interest behind this event, so I think (my guess, I have no data to back that up) that most people take vacation days and pay their own ticket.