That graph is two axis. Outside of economic issues, he's more Conservative than just about about 90 percent of the Republicans.
On only the economic issues, he's only more Conservative than 100% of the Democrats, and less than 100% of the Republicans.
So one one axis he's heavily conservative and on the other he's about 50:50.
But that's only with ongoing relentless democratic party pressure. Swing that pressure the other way, and he seems like an easy fit in the GOP.
You are, uh, not reading that chart very well. First, the ‘other issues’ axis is a weird grab bag that isn’t particularly informative, which is why it’s often not plotted.
Second, the democrats and the republicans are not particularly far apart on the other issues axis, meaning the parties don’t disagree on them that much, so it’s not like because Manchin is to the right of republicans on that he’s suddenly WAY to the right of democrats. He’s similarly apart from both parties on that issue.
Third, on the economics issue while he might be to the right of the median democrat he is FAR to the left of the median Republican.
If you download the data you'll see the absolute value of the distance of Manchin's DW-NOMINATE score from the average senator of each party (counting the two independents as Democrats) for the current senate by dimension and party.
1st dimension
Democrats: 0.29
Republicans: 0.58
Second dimension
Democrats: 0.53
Republicans: 0.40
Sum of the two:
Democrats: 0.82
Republicans: 0.98
So long story short as to why he doesn't become a Republican is that he's ideologically considerably closer to Democrats than Republicans. Finally, if you think this is due to 'relentless pressure' by Democrats you should have figured out by now that Democrats have absolutely zero leverage over him, this is in marked contrast to if he became a Republican. Manchin knows he will win the Democratic primary, so Democratic attacks on him only help him in the general election. Manchin would get buried in a Republican primary in West Virginia.