That's because Morgana is technically a mage first and a support second. She has a kit that lets her zone enemies, which can make her a potent support if she can also land her Q to snare someone so long their team reports them for being afk. But she remains, first and foremost, a damage dealer, and those don't tend to make the best supports in the purest sense of what a support does. Sometimes it's about more than their damage potential. She can lock down one person, and shield one ally. Not bad, but not really that great in the grand scheme of things, and it become readily apparent during teamfights where even if she lands that max range Q from across the way, chances are the allies will just group up around the affected player until the duration of the snare ends so she's effectively accomplished nothing as the enemy can still force you to fight on their terms, where their snared ADC can still right click you to death behind his tank wall even if he can't move. Her only form of CC outside of her ult, which requires her to be WAY too close to enemies for too long to get any use out of with a relatively slow going support build, is a single target long range skillshot. Her shield is also only effective against magic damage, which means that it's all but worthless depending on who you're fighting, especially if you're up against an all AD team. Might as well just not even level it at that point.
A Nami on the other hand has her bubble, which is effectively an AoE stun if you're good with it, her heal which can give two heals from one cast if you bounce it right along with hitting an enemy, extra on hit damage that scales with her AP but assists your carry, movement speed buff on any ally hit with one of her spells including her ult which can give her entire team a massive MS boost, and a HUGE AoE knockup that's as good for engages as it is disengages. Everything she does is directly helping a teammate either deal damage, chase enemies, escape bad situations or recovering from damage done. She can't cast a spell without having it support someone in some way and in that regard she is a stronger support, even if Morgana lets you win lane easier by merit of her insanely long snare.
Honestly I'd much rather have a good Nami than a good Morgana any day.