Multiple reasons:
- Mechanically speaking (aim + movement), Valorant is quite a bit easier than CSGO, unless you're playing high mobility agents like Jett and Raze, which you typically don't see the old CS pros on
- Valorant has a lot of emphasis on gamesense and tactics/strategy on pro level, so the CS pros who have years upon years of experience are naturally going to last longer here since the impact of their gamesense is quite high
There is a lot of new talent, and you tend to see them on duelist roles, and you'll see a lot of IGLs that come from CSGO (ANGE1, Ethan, boaster, FNS, ScreaM). It makes a lot of sense.