This make sense, considering that Android L (Android 5.0) is probably the biggest new release since Android 1.0. There's a new runtime (ART replaces Dalvik), 64-bit support, a completely new design language, there are a lot of new APIs and new classes of devices that require optimized interfaces.
"The Developer Preview lets you explore features and capabilities of the L release and get started developing and testing on the new platform. The L Developer Preview includes updated SDK tools, system images for testing on an emulator, and system images for testing on a Nexus 5 or Nexus 7 device." Only the latest of version of Nexus 7 Wifi is supported. "With the SDK Tools, and Nexus device images, you can get a head start on testing out your app on the latest Android platform months before the official launch. You can use the extra lead time to take advantage of all the new app features and APIs in your apps," informs Google.
A yearly major release with a few months of beta testing has a lot of advantages:
* bugs can be found faster and the final release will be better. There's a bug tracker for the Dev Preview
* Nexus phones and tablets are actually used as developer devices
* both iOS and Windows Phone allow developers to install early releases. Android is now more developer-friendly
* developers are expected to test their apps in the new Android version and take advantage of the new features, so Android apps will get better
* developers have more time to read about the latest features and to update their apps
* developers and power users feel important because they get to try the latest features faster
* there's more hype about the latest Android release: journalists and bloggers write more articles about the new features because not everyone wants to install "dev preview" software
* phone manufactures have a few more months to integrate, optimize and test their own software
* when the new Android version is publicly released, more devices will be updated (not just Nexus and Play Edition phones and tablets)
* new Android versions will get significant market share faster.
To sum up, Android developer previews are better for developers, users, phone manufacturers and for Google. Developers, manufactures and Google have more time to improve their software, while users will install a better OS and higher quality apps.