Mentality and offensive quality.
The mentality of players like Bernardo, Stones, Sterling, Mahrez even Gundogan is just not that strong. They are not weak, but you just don't feel they thrive under pressure and that they will be the best player on the pitch on the BIG games. Players like KDB, Dias, Foden, even Ederson, Zinchenko or Walker tend to play really well on big matches.
The main thing is quality, specially for an offensive team like ours. We have a lot of depth and great squad players, but other than KDB which player is without a doubt a top 15 player in the world? Foden and Dias can get there if they keep this season's level. We have a bunch of top 15-50 players, but against elite competition we should have 1 or 2 more offensive top 5-20 players ( Haaland, Kane, Coman, Gnabry, Kante, Salah, Sancho, Verrati, Kimmich).
If we want to win PL(which is great) what we have is enough.Good quality and a ton of depth. But against the elite in cups competition, you need stars.