Why does a spinning top not stop when you touch it in space?

This is not true a spinning top would stop if you touch it in space just as it does on Earth.

The key point here is that if the top is spinning in a vacuum (where there is no friction with air or other gases) and then you touch it with your finger, you are transferring some of your momentum to the top, which will cause it to stop spinning or change its axis of rotation.