My advanced theory:
In 2020 rallying stopped, lot of (co-)drivers did something else and now when rallying resumed and they went back they realized they now prefer "something else". (children, education, business w/e....) or they now don't approach rallying with same commitment level (and drivers don't like it).
In case of drivers you don't see it stand out so much, you just simply don't hear about them. In case of codrivers it's much more public cause drivers stay with different codriver.
Also co-drivers make much less money than drivers, so it might be more tempting to do something else. (or demand higher salary like Gilsoul seemingly did)