If you already have directx 10 installed why does flight simulator x say you need to install directx 9.0?

Direct X 10 is a completely new Direct X. It as far as i know has nothing to virtually nothing in terms of files from Direct X 10. By installing Direct X9 it will NOT overwrite your direct X10 but what it will do is add the direct X9 files to your direct x and increase compatibility with software that requires the components that belong to DirectX 9. Hope that was helpful