Why is face id only available on ipad pro 11 inch and up?

The hardware needed for Face ID is only installed on the iPad Pro 11” and up devices. The iPad Air does not have the required camera hardware for Face ID and therefore uses the Touch ID which has the finger print scanner in the Home Button. The versions that use Face ID do not have Home Buttons and therefore have no way of doing Touch ID so they use Face ID instead. There is nothing that you can install to get Face ID to work on any other versions as the hardware will NOT support it.

No, it does not have the required hardware.

Because Apple didn't include the required True Depth Camera for Face ID to work until iPad 11 and 12.9 was introduced in 2018. There is no way to make an iPad which does not have the required True Depth Camera Face ID compatible.