My guess is that the map's crucial information is sever-sided. You have the necessary files so you can visually see the map on your computer, but you can't change anything because your only modifying the files that are only on your computer. Changing them will only disrupt the information that needs to be sent to the sever and back to you.
You can modify other things like guns, crosshairs, etc.. You will be able to see the modify version because the visual files are stored on your computer, but no one else will see the difference.