In the first place it's because that broks IGT timer (Main timing methood btw). It becomes slower. Everything including Hitman's move speed becomes slower you can tell. Yes but it's not fair because gameplay is easier with slowmotion

All runs with more than 60 FPS will be remooved or retimed by RTA soon

Thank you for your attention

update: after some thinking it was decided not to remove high fps runs from leaderboard runs with high fps will be retimed by requests (if someone will not be able to beat current result on LB because of FPS)