I think the bigger factor though is that the novelty of the new Wembley has worn off. Let's be honest, our last two visits have been (a) recent and (b) crap - and if you're not that bothered, spending £££ on another potentially crap day out that you've done twice already probably isn't that appealing.
This is exactly what it is in my experience of talking to people who aren't going this time, but have been before.
It doesn't matter anyway. We take 30,000 we aren't going to suddenly get 15K at home each week. The 20K who are there this time may make our more regular support less diluted.