I've heard foreign fans that don't know the songs and are too busy taking it all in. I've heard it comes to the working class being priced out. I've heard the lack of standing. I've heard a lot of reasons. I'll defer to the season ticket holders to say what all drives it, but it's obvious to just about everyone everywhere that the atmosphere as noticibly slipped.
I think that the people attending football nowadays generally care less than they would in the past. People have far more competing interests in the age of the Internet. There was a time when for most match-attending fans, going to the football would be the highlight of the week, the cornerstone of your social calendar. For most middle class people, it is just another piece of entertainment. Something else to consume and document through social media and selfies. I have lots of other interests, but going to the football, walking to the stadium through the streets of ex-council houses, is the thing I love most about living in Manchester. I genuinely care and am always hurt when United lose, but, honestly, most other people I know who support United are kind of indifferent to being a fan nowadays. They'll jump on the train during the good times, and tune out during the leaner times.
People always suggest lowering prices to get more young people in, but the majority of young people around me in the Stretford End are the ones who never really sing and chant. Short of carving out a tifosi/ultra section in the stadium in the medium term, I'm not sure you are ever going to get a proper, consistent atmosphere back. Modern football is not conducive to this though. The Premier League is built around a sanitised global product where your worth directly correlates to the amount you are willing to spend. Global is the new local. Is it any wonder that many of the best atmospheres are in provincial locations, both domestically and on the continent, where clubs are still the primary representative and focus of the town or city?
My thoughts and solutions on the matter are not entirely clear, but the "problem" (if there even is one depending on whose perspective you look from) is only going to get worse until this era of football shifts.