Same reason so many pubs are closing, people stay at home and drink. People stay at home and watch Sky. There was a time when homes didn't have central heating to keep you warm, a fridge to keep your beer cold, and they had a black and 12" TV with two or three channels. They had lino on the floors and an outside toilet.
People used to go out because it was often more comfortable than staying at home, it was often cheaper to go sit in the pub than pay to burn coal in a house with ill fitting windows and no electricity because you'd run out of shillings for the meter.
Then we all got fridges, carpets, 30" flat screens, super fast high speed broadband, inside toilets, bathrooms and lots and lots of credit (or to give it it's real name...Debt) hey presto every body stays in and got out of the habit.
Now if you want to go out you jump in the car and the Worlds your oyster, dozens of sporting venues with in an hours drive, country side and other leisure activities
And people moan about poverty, I wouldn't want to go back in time to be honest even though there were very few burglaries when I grew up (that was only because people had f### all to steel). I prefer it the way it is now.
What to do about falling attendances? God himself only knows, but I suspect if clubs don't identify and work with the trends then they wont be around for long, and that's not having a pop at Oldham, thta's all clubs.