A few observations I'd like to throw in.
1) While everyone is vying for the K-12 demographic, shows that air on Nick tend to be more towards the K end, shows that air on CN the 12 end, and shows that air on Disney in between. CN has a lot of shows that are popular with young teens and also Adult Swim, which brings in a lot of college viewers. Many of these see Nick as the more babyish channel, and dismiss any new shows as such, typically over the internet which they are much more versed at using and getting their opinions out than a child. So, while Nickelodeon gets a larger amount of viewers, these make up a silent majority and the perception that Nickelodeon is a "dead", "dying", "stale" channel has been around for as long as the internet has been popular, believe me. I can vaguely remember on some old cartoon forum I browsed people thinking Rocket Power would be the death of the channel.
2) Others have stated, overuse of the main bread makers, Spongebob and FOP. While I resent some have insulted these arguably classic shows as "trash", such a moniker is at least fitting for the latest season of FOP which was one of the worst cartoon productions I have ever seen, I barely watch Spongebob anymore at all but the opinion doesn't seem to be any brighter from what I've read. Those lists that have been circulating the internet which show Spongebob dominating the top 50 ratings have looked ridiculous for the channel, even when one considers that with Spongebob removed, Nick still had a decently diverse lineup but little of it was able to even crack 2 million viewers. It just gives naysayers even more ammo to use against this channel when it can easily be perceived to be nothing but the "Spongebob channel".
3) The same guys stinking up the joint for years. Mainly, Hartman. He's regressed so ridiculously far as a cartoonist in the past seven or so years. Once one of the giants in the industry, stupid decisions for his shows like Poof, Sparky, the influx of comedy into Danny Phantom, live action movies, bait-and-switch finales, it's all reduced him to midget status and he has become a laughing stock. Also, even though he hasn't worked with the man in years, Hartman is sometimes named as a co-defendant of Seth MacFarlane, another man who has become quite reviled by internet communities (Because South Park said he was bad... or something). Another thing Hartman did, which probably can't be mentioned here, and I'm not even sure I put the blame on him anyways, has made Hartman an enemy of hardcore cartoon fans. You'll have to ask around elsewhere if you want to know about that though. It's not just Hartman of course, Dan Schneider has also lost his magic.
4) Bad online marketing. CN and Disney have simply done a better job getting their product out their on the internet, while Nickelodeon up until only recently simply relied on their horrid flash site where bots posted most of the praising comments. They've been making some progress on Facebook and Tumblr, but compared to what Disney has with their Gravity Falls website, they've got a long ways to go.
Those are just guesses though. The biggest factor simply is that Nickelodeon has been struggling to produce another clear cut hit. The Avatar series was the real last one, and before that, FOP, AGU, and Spongebob. While Sanjay and Craig gets good ratings, I'm hesitant to call it a hit, we saw how fastly the TUFF Puppy ratings faded. Korra, in my opinion, has been a little disappointing compared to its predecessor. I don't see Breadwinners being the newest Spongebob either. They've got the right idea by doing things like, say, not picking up any of Hartman's pilots, and hiring from new and unique talent, so we'll see how it pans out in the coming years.
