Lots of reasons - time of year; august bank holiday many families on holiday. Grand final has overtaken CC final as the premier final. Magic weekend has also been added to the calender as a jamboree that all fans get on board, something that used to be reserved for the CC final. Poor marketing and scheduling by the RFL for the rounds.

They are just a few of the reasons i can think off the top of my head. My personal opinion is that the new wembley just does not have the same feel as the old which was an absolute unbelievable stadium and atmosphere. That has not been recreated with the new and is basically Wembley in name only. I thought the millennium stadium was as close as you get to the old wembley in modern times. 70 odd thousand is about where we are at and it would be full house most years.
I get this may not be a popular view and many would prefer to stick it out with Wembley but i find it hard to see us improving too much on the 60 - 70 k crowds we have been getting for a few years now; simply put the Grand Final is the pinnacle now and the days of 80k plus finals are long gone.

Doesn't matter if it's crap or not, that's how they feel and as a game, we can't afford people feeling like that.