Android's foundation has been mostly stable for years now, with fairly minor changes between releases. So I guess they just don't want to deal with too many versions to document and support, given that device vendors are generally awful.
Also for a long time they were doing yearly (or longer) release, afaiu it's only the past two years that they switched to quarterly (with the QPR release).
Also for a long time they were doing yearly (or longer) release, afaiu it's only the past two years that they switched to quarterly (with the QPR release).