Why is that?
I really liked his first few movies.
"The Sixth Sense" is a classic.
Plus having "Night" in your name is cool.

Yea, Sixth Sense was good. Unbreakable was decent. Signs was bearable. But (here's the "why"), then you have:
The Village (crap)

Lady in the Water (unbearable crap)

The Happening (somehow he "topped" Lady in the Water and made a movie that was even more garbage)

Last Airbender (somehow he managed to sink even FURTHER, this time not even dealing with his own material. well...he wrote the screenplay, but Avatar isn't his creation. And what do we get? one of the lowest rated movies on Rotten Tomatoes. Complete and utter garbage)

Devil (then he tried to let somebody else direct as he just produced and wrote the story--not the screenplay, just the story. and yet again critics are panning it left and right--i refuse to watch it)

So as you see, even since Sixth Sense he's been getting worse and worse. After Signs he completely fell off, even managed to pump out worse garbage each time. I lost ALL faith and cut him off after The Happening, and started losing faith in him after The Lady in the Water.

By the way, I only assume when you say "first few" that you consider Sixth Sense to be his first, like most people do. But, he did have a movie before Sixth Sense called Wide Awake. Guess what...laughable at best (and that's laughing AT, not laughing WITH the movie).

I enjoyed Sixth Sense, Signs was OK. As for the rest of your movies...F*CK YOU, M Night...F*CK YOU