Because MSVC.
I have attempted to add that overload in the past, but it just confuses all of the older MS compilers, which use a typedef to define size_t as int, so it thinks you’re multiply-defining the constructor. And IIRC trying to use #ifdefs to only enable the overload on proper compilers caused other knock-on problems… It was certainly a total can of worms, I’m afraid. One day, when everyone’s on VC2010 and later, I’ll bring it back.