Obvious reason is personality, Jamppi is a natural leader and if I was a top player I would do everything to be on a team with an IGL like him, an IGL like Jamppi together with PAL who is also great is a perfect recipe for success, IGLs like that are worth their weight in gold both in CS and VAL.
Another one is the way they see the game, Jamppi is a very agressive player who likes to go in first and sacrifice himself even on initiator (not in a dumb way like Ange1 tho) and the way he calls is more explosive, fast, a lot of playing together as a 5 man deathball or 4-0-1 compared to Sayf, I think Sayf has a good tactical mind as we all saw the excellent macro and calling in KickOff but he is too readable, when you play a slow spread default every single round then everyone knows what to expect from you and you will get punished internationally, he also seems to be a player who focuses a lot on giving up space and playing retake or playing together at the expense of map control/info, I think this swedish style of calling doesnt work that great nowadays in both CS and VAL, specially in an arena enviorement where the games can become very emotional/momentum based and the sound is dogshit so you need to be adaptable and thrive in the chaos, sadly I always enjoy watching Sayf streams and I can say he is a bit susceptible and almost petty at times so I dont think he is the type of person to be humble enough to realize whats bad in how he approaches the game and genuily change his passive ways. He strikes me as the type of guy that will die with his fetishist way of playing valorant, u can even tell from how he peeks/holds angles that he isnt comfortable with being agressive/chaotic. If I could tell him one thing it would be to become a more pragmatic caller and stop trying to play perfect fantasy valorant and trying to make all of us fall asleep by entering site with 10 seconds left for 12 straight rounds. Not even the best IGLs in CS play like that in a game that rewards being passive way more than valorant ffs