Because ZSS is able to control the pace of the match easier than Falcon. ZSS can zone, Falcon less so. She's also a lot less fragile defensively, and its questionable that CF is more mobile than ZSS, who has access to better wavelands, a dive kick, two midair jumps, (and less importantly) b-reverse whips/blasters, super upB, and crawl.
Also, both of their combo games are comparable - spam a bunch of aerials into kill moves. ZSS' is a lot less regimented, but she also can use tilts and smash attacks to combo whereas Falcon doesn't really have that option.
Her access to more options to convert from neutral and arguably better mobility gives her a case for being better than Falcon, especially in the neutral game. Against Falcon you look out for nair and grab in neutral generally. There's more to manage against ZSS (specials, jump baits, tilts, DD grab, shffls, actual CC options, etc.). She gives up Falcon's ease of killing for this neutral game, and also better vitality in nearly all measures (tech rolls, options out of tech in place, ease of getting out of juggles, recovery length, recovery safety).