I have to say when you get people like my quite big 22 yr old son, singing it and saying how much he loves the movie?
You have to ask...yes 'why' is it.
I thought about this weeks ago, and had to come to the conclusion it is the songs and content and the singer
The story is pretty much in my opinion a good movie but nothing too different.If you watch it, with no Sound!
Only when the sound is introduced, you find that is the key.
The happy-sing along-great lyrics.
That is what has got people in....I believe.