First of all, melee Q has no % HP damage, that's his E knock back. That's the reason his all-in kill potential is gimped unless he gets consistent poke (using E for the nuke is too risky in an all-in most of the early game because it knocks them away from his reliable damage). Melee Q also is not a guaranteed hit, it is unit targeted but after casting it switches to ground targeted (meaning you can juke out from the impact cuz his code is weird like that).
The difference is two fold: Jayce is the only AD one of the three (meaning he's effective just AAing for damage at range, unlike Nid or Elise), and the way they play out that poke>engage pattern is very different, for the reasons I mentioned. Nid and Elise both want to drop all their ranged stuff and then immediately go in during a gank, and they need to go in melee for any real impact since so much of their power is shifted to their melee forms (powerful executes and guaranteed damage as opposed to unreliable damage and utility in ranged). Jayce's issue is that too much of his strength is in his ranged form (EQ is too fast and has too broad of an AoE, even if he would whiff the shot it still explodes to clip enemies) and his melee form lacks the reliable burst of Nidalee's or Elise's. Together this encourages him to sit back as a ranged caster outside the rare opportunity to engage, much more like old ICBM Nidalee prior to her rework.
To fix Jayce they'll need to make his Q work like Karma's Q (only explodes in an AoE if it hits something with the initial skill shot), probably reduce the missile speed with and without his E, and then shift around the damage of his melee form to be a bit more encouraging of offensive play instead of being so defensive and safe.