Question

Why is using kilometers a problem in space

Answer 1

In space you don't use kilometers you use light years. Space is infinite so it would be impossible to use kilometers.

Answer 2

Using kilometers in space is useless, as the distances between objects is drastically incred. A lightyear is 9.5 kilometers. It is simpler to say "Alpha Centauri is 4.1 light years away" than it is to say "Alpha Centauri is 28.95 Trillion kilometers away"