There are a lot of things that contribute to that block falling into "instant classic" territory. Off the top of my head:
1. The play started a good distance away from the basket, with Stewie out near the side of the 3 point line, and Kiah out on the foul line guarding someone else. The spacing makes it elegant to track the progress of the players over the course of the play.
2. It has an easy narrative. "Stewart was being guarded by Tina Charles on the 3 point line. She pump fakes and goes by Charles like nothing. Streaks in for an open layup, when all of a sudden,
boom!"
3. Stewie's attempted release point was quite high. Not quite at rim level, but approaching it. Some of the less athletic centers out there would struggle to get their fingers on that ball after it had gotten that high.
4. Kiah not only managed to reach the ball, she got her whole frigging
hand on it, as if she were blocking some Moriah-sized player instead of Breanna Stewart.
5. Kiah's physical alignment was perfect, such that (in combination with her strength), her balance was barely even disturbed by the whole thing, and she returned to the ground fairly casually.
6. Breanna, on the other hand, bent like a reed in a hurricane.