It's due to multiple factors. Even though ZSS lost a major kill setup, she got multiple buffs that help offset it and giving her an overall better neutral. Plus, the ultimate engine really benefits her, especially if she's playing a more hit and run style and aids her mobility as a whole.I dunno. still managed to be up there despite losing her SSB4 d-throw ladder combos which were big reason she was top-tier im that game. ZSS got lots of direct and indirect buffs in the transition to Ultimate to where you need to play her differently
She has consistent punishes off tech chasing now with her down smash, jab locking with a frame 1 jab and being able to do them out of a dash . Also boost kick working consistently is more beneficial to her rather than being inconsistent on skinny bodies/smash 4 dlc even if it killed off the top earlier, since it works better now as an oos option.
She also still has consistent follow ups off zair, Nair and bair (this one at low percents). Her newest tool in this game though, is her fair. It has an amazing scoop up from a short hop and she can follow up off falling fair1 (even at high percents) which really helps against smaller characters.
She also gained a safe on shield mid range poke with side b that can both shield poke at close range, combo and kill along with less cool down on her neutral b. This allows her to have more pressure options at mid range as well as covering ledge options that she mainly used with zair back in smash 4.
I think what may hold zss back right now though, was alluded in Esam's video about her. While she now may be winning neutral more often, she's not getting crazy damage output from each interaction. So the player will rely on getting reads, early conversion to spike, killing oos and edgeguarding etc. However this type of constant interaction and engagement may need to happen frequently in order for the zss to get the kill (unless she gets an early spike/gimp). During that time, she may be putting herself more at risk as the match progresses, even if she has down b for escaping.