You can also look at it as the casual fan (and even some more than casual) can't afford 2 trips to Cardiff and are banking on an upset. I think if we were playing Leicester in this round there would be the same problems with ticket sales.
I have the resources to go over there once this spring, and decided to take the risk of us getting to the final instead of coming over for the semi. Even for someone coming from Blackburn, 2 trips down can get pricey, factoring in tickets, transportation, food, booze, hotel, etc.