lit-html and LitElement are products of the Polymer project. They're different enough from the classic Polymer library, yet can be used interoperably with Polymer because it's all just web components, that they deserved new names.

The classic Polymer library is in maintenance mode now. It's getting critical bug, security, and performance fixes only. We're moving customers from Polymer 1 & 2 to Polymer 3, then from there we're encouraging them to incrementally adopt LitElement, which again, they can do because they work together.

Our situation is quite different from frameworks because of web components. First, we care more about web components adoption than any specific library. Then, because apps can mix web components made with different libraries, we don't have to bank everything on one flavor or specific set of ideas. That said, we think lit-html is an even better solution than what we've had before, and we're focusing on that.