I would imagine it’s due to the contrast/exposure of the image.
In order to keep the spacecraft and the earth and moon from being completely washed out in the image you lose the visibility of the stars. The sun is extremely bright in space.

Remember there's no (or so little as to not matter) scattering of light in space. Almost all the light you see comes out from the sun and reflects off the three objects in the photo. Earth, Moon, spacecraft. The spacecraft (obviously) is closest and very reflective, so it's going to be the biggest light source in the image. That bright source will determine what exposure is required to be able to pick out the detail they can. A longer exposure would wash out the image and a shorter one wouldn't pick up enough light to see the Earth and Moon in any detail.

If the camera was pointed out away from everything and a photo taken with the correct settings, you would see stars. For example this photograph taken from the ISS.

Just not enough light. The ship, the moon, and the earth in this picture are all pretty well lit up by the Sun, so it’s a pretty quick snap.

Man I asked this question on a different picture last week and got downvoted for being curious lol