Because it’s the lunar daytime.
Lunar daytime is quite different than terrestrial daytime since the sky is still perfectly dark. You could take pictures of the stars during lunar daytime by shielding the lens with a simple baffle.
However, when you expose for the brightly light ground and the objects on it, the exposure value is too low to show the stars. It’s analagous to taking a picture in a brightly lit stadium at night on the earth.