The two other comments here are spot on, but there was also a lot of movement of even former dedicated communists from the East to the West after the wall came down as I once talked about through the lens of my German family in this post.

But it’s a combination of left defeat, lots of the former Eastern left dispersing across the more prosperous West of the country, and the fact the state spent many years enjoying putting the boot in to the East both politically and culturally (although the fash sometimes overstate that aspect as opportunism too).