why is the sky a darker when you at a higher altitudes?

Answer 1

The color we see in the sky is from light scattered by the atmosphere above us. As you go to greater altitude, there is less atmosphere and consequently less scattered light.

Answer 2

It is probably more a direct matter of light pollution and humidity than elevation.
The lower your are there will very likely be more sources of artificial light thay will reflect into the sky and be re-reflected from atmospheric dust and fine water vapor mists. This will cause the sky to appear a little brighter and the stars a little dimmer.
The higher you go the less atmospheric pollution there is.

Answer 3

space is black but the sky looks blue because the air refracts/reflects the light coming from the sun
at higher altitudes there is less air between you and space so it looks closer to black