Uniting for Internet Freedom: Tor Project & Tails Join Forces | Tor Project
blog.torproject.org
The Tor Project and Tails have joined forces and merged operations to streamline operations, expand outreach, and strengthen both organizations' ability to protect people worldwide from surveillance and censorship.

TL;DR:

Incorporating Tails into the Tor Project’s structure allows for easier collaboration, better sustainability, reduced overhead, and expanded training and outreach programs to counter a larger number of digital threats.