Nexus 5 Launcher is NOT part of Android!

I find this strange.

For some reason, Google decided not to include the nexus 5 launcher into AOSP. It seems to have been made solely for the Nexus 5.

Why would Google do this? The new launcher seems to be one of the biggest features of the Nexus 5's software, and seems to fit the design changes made in Kit Kat. The AOSP kit kat launcher looks exactly like the old one from Jellybean. This just confuses me