No other explanation makes any sense.
Certainly there is no supply problem that could keep Apple from introducing a white iPhone. But what if white is a Verizon exclusive in the USA? That would to a certain extent pull the branding rug out from under AT&T and even put a bit more oomph behind those iPhone users who might choose to jump carriers.
It’s silly, I know, but as Mrs. Cringely always says, “Husbands die every day.”