Okay well. This should be obvious, but what I'm about to say is purely my opinion.
If you've seen the trailer for little Mac where Samus makes fun of how short Mac is by measuring his height compared to hers. You could think of that line she draws as "you must be at least this tall for zss to stand a chance against you"
In case you've never witnessed or been told this, zss has a hell of a time dealing with short characters. Why? Well just look at all of her moves.. You'd struggle to find an aerial or tilt that is active for more than like 2-3 frames.
Besides this, her offensive Hitboxes are actually fairly small as well, since there is a sizeable gap between her and the whip. People often think nair is a huge hitbox, but if you actually look at the size of the hitbox that is actually low enough to hit a walking/running pikachu.. It's really really small. A Pikachu would have an easier time reacting and punishing the nair attempt since zss has to wait longer in order to get low enough to hit Pikachu. And again, since the hitbox is really small at that height, it's just hard to hit a moving Pikachu in general sometimes.
That pretty much sums up the matchup with any character below the height of like pit. Pikachu just happens to have all of the things that zss doesn't like: small size, fast speed, amazing edgeguarding that can challenge her recovery, and a decent upsmash to punish her jump attempts.
