And that is because the right or the left moves further away in the political space? By definition it seems to be the right-wing moving more extreme rather than non right-wing all moving together towards the left.
Entryism. By definition it seems to be the left-wing moving more extreme rather than non left-wing all moving together towards the right. Democrats have shifted more to the left than Republicans to the right: https://archive.is/IILDt
"All organizations that are not actually right-wing will over time become left-wing."
https://en.wikipedia.org/wiki/John_O%27Sullivan_(columnist)#...