Why is the electron configuration for copper 1s2 2s2 2p6 3s2 3p6 3d10 4s1 instead of 1s2 2s2 2p6 3s2 3p6 3d9 4s2

In electron configuration, there are 4 sub-shells S, P, D, that hold a certain amount of electrons.
S - holds up to 2 
P- holds up to 6
D- holds up to 10
In order when doing electron configuration, each sub-shell must  be fully filled before moving onto the next one. That means that if you only have 1 S, you can't move onto the next  sub-shell. In this situation, the 3 d 9 only had 9 of the 10 needed to fully complete its shell. You wouldn't be able to move onto 4 s just yet. This makes the first representation of correct since both shells are completed correctly.