It's all about Howard:

He didn't want a fourth judge to take air time and attention away from him.
The new judges are younger that Howard and will point out his lack of revelance and understanding of the youth market.
There is no promotion or build-up from NBC like last year and any promo that's coming will focus on the new hires.
The New Orleans auditions were a disaster, Howard was booed.

And most importantly -

He wants to distance himself from this year so that if it tanks he can say it's because they didn't do things the way he suggested.