There's no word on why the Smart Battery Cases are temporarily unavailable from Apple, but there could have been a manufacturing delay or issue that has caused available supply to dry up.
Right now, Smart Battery Cases for the iPhone XS, XS Max, and XR all list delivery dates between July 1 and July 16, with pickup available in retail stores on July 16.
Those dates pertain to all colors, including Pink Sand, White, and Black, for all Smart Battery Cases.
The Smart Battery Case shortage also means that warranty replacements for Smart Battery Cases in need of repair are unavailable at the current time, as MacRumors reader Andy learned from a retail employee. Replacements will likely be available again in July when supplies improve.