Avalibility (job being the majority one) and cash would be the two that prevent people from watching their club every week. Throw in the legit Dortmund fans in Britain, then the ones who are just looking for a ticket to watch the game. I can't imagine those can combined can amount upwards of 400'000 fans however. So there is a point of a decent amount just there for their special day, then in a few years, they'll barely remember who the first team was. The lack of avalibility is the biggest key point though, they'll most likely try to book a day off work to watch.
Definately, when you watch a cup final in Scotland, you'll see crowds in the tens of thousands. 3-4 days later for their league match, back down to the usual number. Frustrating experience to see, but nothing one can do about it. Also happens in the league playoffs in England, and it happened at that FA Trophy a few weeks ago, from memory.
I would like to hope it's what Dortmund are going for. They are a fan ran club first, they won't (hopefully) turn away their regulars just for a couple thousand who'll pay upwards of £500 more.
I find it hard to believe that the German Dortmund fans would struggle to find time for a regular home game in Germany but would have the time to go to England, at best that's a case of fair weather fans coming out of the woodwork. As for money, Bundesliga tickets are dirt cheap, a trip to England for a Champion's league final (travel/hotel/match ticket) would be really expensive. When you consider all of that it doesn't make sense that the club would suddenly be completely overwhelmed by ticket demands from genuine supporters. Maybe a few people fit your examples but I doubt it's really enough to catch the club by surprise.
The "decent amount just there for their special day" who will "barely remember who the first team was" in a few years sound like bandwagoners to me. I'm not saying that none of the extra 400,000 are genuine fans, but there must be a fair amount of cling-ons (no star trek) looking for a ticket amongst that lot, that's all I'm saying.
When we reached the division one/champs play off finals in 2001/2002 we took 30,000, despite only regularly receiving around 16,000-20,000 at Carra Rud at the time. Makes you wonder how people can muster up the desire to travel all the way from East Anglia to Wales but not to turn up regularly at home games. We often had spare seats during that season so you have to question where those other 10,000 "fans" were on a normal basis. Proper fair weather fans/band wagoners.
If Dortmund don't have a ballot system then it's a bit of a fuckern jewk to be honest.