Because BBC reports him as saying:
"Mr Osborne urged top-rate taxpayers to stop claiming child benefit altogether, saying this would be the "most sensible" thing to do."
Well no, actually, it wouldn't be sensible at all if you are not working at all/enough to meet your National Insurance contribution, as claiming Child Benefit ensures you get Home Responsibilities Protection (effectively, a full contribution for that year on your state pension).
So please, nobody stop claiming Child Benefit without checking you're not going to be stuffing up your State Pension first.