What is Google Waiting For a Stable Version of Gmail?

Gmail is out there for years and works fine but it is still in Beta. Gmail team improved Gmail Spam very well and added lots of features including themes support. Even though, they put delete button !!! However, it is still in Beta. I don’t understand how Google’s applications/services versioning works. Sometimes, I though they may still dealing with annoying security threats or working on a new spam filter to make super duper unbreakable spam detection and removal system to reach stable version of Gmail. But, Google isn’t perfect and Gmail too.