No, travel costs are not a purely geographic thing. London is an unusually central location for the world's airlines, and also has a number of budget airlines with many routes. If everyone was driving across Europe to Wikimania then it would be logical to hold it somewhere near Crackow or Bratislava. But if most people are flying and many are doing so from cities that don't have major international airports then London is unusually, perhaps uniquely well connected.

Flying costs vary somewhat with distance, but more with competition, number of connections needed and standard of airline. There are disadvantages to budget airlines that don't allocate a specific seat and charge extra for baggage in the hold and even food, but they are usually cheaper. I know a family that have decided never to fly RyanAir again after not being able to get a block of four seats for two adults and two small children. But it is very cheap for one adult flying alone with just a weekend bag and no preference as to where they sit, and I suspect that describes the typical Wikimanian.