Stars are dim. Earth and the Moon are bright. If you exposed the shot such that stars would be visible, the Earth and the Moon would be horrendously overexposed.

If you look at this one of the moonlit nightside of the Earth they took on the way out, you can see stars. The website has EXIF-data on the bottom with more info on the exposure.

Just the light contrast. Stars are not as bright as the light from the sun reflecting off the Earth and the Moon. If they adjusted the exposure enough to see the stars, the Earth and Moon would look blindingly bright in comparison.

It truly needs to be very dark for stars to be easily visible, to the extent that the mere lights from a nearby city shining through the atmosphere are enough to render them invisible on a clear night.

Others have answered, but here's an extra bit of trivia: space looks like a deep dark void with these two in frame...

...turn around, when the sun and planets aren't interfering and you see the stars. An absolute sea of stars that space appears full of them. Apparently it's pretty damn fantastic.

Edit: here's one now: Starstruck

Thanks!

Yes. I offload all my thoughts into my Keep. I can then use it to see what i was thinking about in the past

Honestly I wish someone wrote an OS clone. The UI isn't all that complex

There's an XKCD for everything. How do they know?!