He probably saw the writing on the wall. Justice is the clear favorite for the seat. This pretty much seals that this seat will be lost to the GOP. I don't see a Democratic candidate having Manchin's success.
I will be interested what this means regarding No Labels. There was some talk of Manchin maybe being No Labels' pick for the presidency.
The 2024 Senate map is brutal. This seat was one of the least likely to hold, but still this narrows the path for Dems to hold the chamber. I expect the Dems to retake the House, but I don't know about the Senate unless Republicans pick really bad candidates.