I think you're right. Despite being bang in the middle of Europe, it's a quite difficult place to get to whereas, Lille was accesible by car, by Eurostar, by plane to Paris or Brussels for example. Wolfsburg was accessible from all over the UK via plane to Hanover, Berlin, Bremen, Dusseldorf, and i know a few who drove, upon arrival it was pleasantly cheap, and the incentive of our first European games were on display.
With Switzerland and Young Boys, we find that it's peak in ski season, so hotels are expensive, flights are expensive and is an additional 755km and 781km each way from Lille and Wolfsburg respectively, add that to the £6 a pint, and the fact we're playing megagash, and it's a hugely uninspiring trip, whereas something like Inter i believe would have drew a close 10,000.