User blog comment:Craftyfirestorm77/Summoner Spells on Fizz/@comment-11322683-20130715122950/@comment-3476718-20130715125929

Like most AP assassins, one of Fizz's few weaknesses is his horrible early game- especially against mids who can survive his harass early game and zone him out of farming completely after that- mainly shield champions (like Morgana), who can wait for the burst and counter with one of their own, or ridiculously long ranged/safe champions (like Xerath) who don't need to worry about their laner much and farm/harass all the way up to level 6.

Besides, most AP mids will probably back off after the first combo and try to play it incredibly safe after that- something else that counters Fizz early game. If he can't get kills, he can't roam.

Come to think of it, though, it's probably why Fizz has that passive anyway.