Apparently some "insiders" told SamMobile that both the Galaxy S5 and Galaxy Note 4 will receive the Android L system update by around the end of November or in early December. They also said it's not clear when Google make Android L available for Nexus devices, though if the time frame holds true for Samsung's handsets, it shouldn't take long for Google to update its own hardware.
The reason you typically have to wait for your Android device to receive a software update is because it's a multi-step process. Obviously the first step is the release of the OS itself, which Google and its Android engineers are in charge of. After that, it's up to both the handset maker (Samsung, for example) and wireless carrier to customize and validate the OS, both of which can take some time.
Android L represents one of the biggest updates to the Android ecosystem in some time. It will bring about a new UI design, a new runtime called ART (Android Runtime), enhanced notifications, increased efficiency, support for OpenGL ES 3.1, an improved camera API for advanced camera capabilities, and lots more.