Most users have migrated off of v5 by this point, and I think a year back is far enough for us to support.
Supporting old versions restricts which APIs we can use, and makes it harder to build extension rules as we have to take care to fill in missing messageIds ourselves.

ESLint v6.0.0 was released 21 June 2019 - over 1 year ago.
ESLint v5.0.0 was released 22 June 2018 - over 2 years ago. This version predates the official forming of this project.