Because primaries are base elections. The base of the Republican party is far more conservative than Manchin ever will be. A more conservative candidate will mop the floor with Manchin.
Who is waiting in the wings to take him out?
The Attorney General Patrick Morrisey ran in the 2018 General Election against Manchin and lost 49.6 - 46.3. The Libertarian took 4.2% of the vote.
In the 2018 Republican primary election, Morrisey got 34.9% and the runner-up Evan Jenkins (Supreme Court of Appeals WV) got 29.2%.
I don't think it's a shoo-in to assume that there is a dark horse waiting to take out Manchin in the primary, but I could be wrong. I do think his opposition to the AOC-Sanders agenda will win him points in West Virginia as a Senator who can get things done, where a novice will be at the back of the line.
-PJ