Lit's been around, starting as lit-html and lit-element, since 2017. Chrome, Firefox, Photoshop, Reddit, and more are built with it, so it's definitely not a library of the week, it just has less hype than others.
Now compare it to React, Angular and Vue: the 3 of them had a complete change of course at some point and lost a good amount of documentation and ecosystem while having to maintain both for some time. That never happened with Lit (although it seems to have being created by the same team that maintained Polymer).