Because it provides context on the intent of the challenge. His foot changed direction after making contact with the ball. Upon contact, Casemiro’s bending his leg at the knee, further adding to the mitigating factors in assessing the force and intent of the tackle, which are judged to ascertain whether it’s a red card or not. The laws do not specifically state anything about studs on opponent = red card, but usually studs up suggest recklessness or intent. But as we’ve seen multiple times this season (and even this weekend), you can tackle someone with your studs up no problem without being shown a red card, but again it’s Casemiro being singled out out of all the players doing something that could see someone get a red card.
It’s clearly not a clear and obvious mistake showing a yellow card for that tackle, so VAR should never have intervened in that situation. Just like VAR shouldn’t have intervened if Taylor had given a red card. But if Taylor hadn’t given a foul for it, then an intervention would’ve been reasonable.