Cassiopeia is in a slightly awkward spot
She needs items to get going and her level 2/3 cheese was nerfed. Bad early game, fair enough.
Mid game? Still needs RoA and Tear to charge up, while
users are running around one-shotting everybody.
What about lategame? She's supposed to be the ultimate lategame AP-DPS hyper carry. Well not really..

all scale WAY harder.
Heck even
is in a better spot because of superior range. Ever been hit by an Ezreal Q or Cait E+ auto lategame? You know how much it hurts as a squishy mage.
Cass doesn't really have a good keystone, Phase Rush is meh and neither Aery nor Comet work well on her. She recently received some buffs but it's not nearly enough. The problem here is that she doesn't really serve any purpose in the current meta.
You want lategame AP hyper carry?
is already a monster midgame. Want a strong midgame AP champion? Just about any other AP mage is a fine choice. Want carries with similar damage output but far superior survivability with a class dedicated to keeping them alive? Pick ADCs.