Exactly for the reason I wrote above, because his definition of "nothing" is NOT nothing. Quantum fields are something.
To make a pompous proclamation like " UNIVERSE FROM NOTHING", then admit that 1. nothing was deceptively redefined 2. that you don't even know for sure. That seems to be more than enough valid reasons for people to mock him, I personally don't care about Krauss either way, if he can make money using such tactics, good on him.
Also "scientific consensus" doesn't mean much when it comes to cosmology, since there was "scientific consensus" on many false theories like the steady state theory. Making such an argument is often nothing but an appeal to authority in disguise, in order to validate fragile ideas.