melee carries don't do nearly as much as ranged ones
not as much safety (obvious)
not nearly as much objective control
no way to siege towers with the way riot's set up the game
generally have worse utility, for some reason ranged ADs (and obviously APCs) are deceptively valuable in utility lately while it's very sparingly put on melee carries - this one's important, prob biggest reason why so many melee carries flop in league's CC-oversaturated meta
only deals comparable damage to current top tier ADCs, sometimes less

not to mention whenever they're good
the community riots -- probably the biggest reason tbh, for some reason there isn't a single melee carry that's not despised when strong so they usually don't stay strong for long
we must come together and demand justice for melee carry lords, they are just as valid as any carry ;(