why cant i download itunes new version?

It may be related to the version of OS X you have on your Mac. The latest iTunes requires OS X 10.8.5 or later.

You can download it. I can download it on my 13 year old Mac. You just can't install it.

iOS9 on a mobile device requires iTunes 12.3 or higher which in turn requires a computer running OSX 10.8.5 or higher. System Update only checks for updates for the current system version you are running but that itself may be out of date. It may or may not be possible to upgrade your computer to the required system:

You can either buy a download code for OSX 10.8 Mountain Lion from Apple online, or you can try installing the free El Capitan OSX 10.11. El Capitan may run slower on older machines and require additional RAM purchase. Doing a big jump in system versions is also more likely to affect old software.

Snow leopard to el capitan, will it cause my macbook to be slow? - https://discussions.apple.com/thread/7412959

Mountain Lion 10.8 purchase link U.S.A. - http://www.apple.com/shop/product/D6377Z/A/os-x-mountain-lion

Mountain Lion 10.8 purchase link U.K. - http://www.apple.com/uk/shop/product/D6377ZM/A/os-x-mountain-lion

Current OSX general upgrade information, including system requirements - http://www.apple.com/osx/how-to-upgrade/

If you have any PPC generation software that has been running under OSX 10.6 it will not be supported under any later system versions.