As many have stated here, there are a number of factors in the NFL's ratings decline...but I'll differ with one, and that's "overexposure"...
College Football is on every channel from Thursday - Saturday in a myriad of time slots...and its ratings have never been higher. You'll have games on ABC, NBC, CBS, FS1, FS2, ESPN, ESPN2, ESPNnews, SEC Network, Big Ten Nework...and others from noon until 9pm... Football is great when it is interesting.
The problem with the NFL right now is... it really just isn't that interesting. The Monday Night Football schedule has been just pathetic (and that's on purpose because the NFL hates ABC/ESPN)...the Thursday night (Divisional) games haven't been good, and you're seeing a lot of mediocre play around the league. It just isn't good football...and people won't watch if it isn't interesting.