I think the common denominator is SKY but along with that there is the 'off-shoots' from the advent of SKY and the Premier League era. The sanitised, sterile, family orientated atmosphere at the ground has certainly stopped a huge number of long standing fans from attending regularly. Other points associated with this such as the ground becoming all-seater, the smoking ban, over-zealous stewarding, the general feeling of not being allowed to be a bit raucous and generally enjoy yourself, the distinct lack of an old fashioned football ground atmosphere have all had an impact. I know if I was a 7 year old now rather than in 1983 and I was first taken up the match, I don't think I'd experience the same feeling of excitement as I did then.
Another off-shoot of the SKY/ Premier League era is obviously the vast increase in the wealth in the game which again makes the game more and more disconnected from the traditional support base. Why would a working class bloke struggling to make ends meet want to help finance millionaire footballers? This is another reason without a doubt. Along the same lines, the increased wealth has obviously attracted more and more foreign players to the extent that English/British players are now in the minority, even at our level and below. Again this causes the traditional fan base to become more and more alienated from their club if they can't relate to the players who are representing their local side.
There are also more and more alternative forms of entertainment on offer now, as others have said, often from the comfort of your own living room, which could easily be far more tempting to pursue on a cold Winter's day. Once the habit of attending the match has been broken, it's awfully difficult to get back. The fact that you can now watch pretty much every game televised, even the non-SKY fixtures is bound to have serious consequences on attendances also.
I think everyone has their own individual reasons for not attending but I think I've highlighted the main points outside of the usual work/distance/affordability issues.