What makes a hero a support is not the heals, but utility for the team.
Every single support hero has a way to help their team along side with the healing:
Ana - Bionade and sleep dart
Bap - Lamp and high damage output
Brig - Frontline presence, armor and shield bash
Lucio - Speed boost, boop (can displace enemies and force cooldowns) and a defensive ult
Mercy - Damage boost and Resurection
Zen - Discord Orb and high damage output
Moira has none of that. The most useful thing in her kit is the fast Coalescence charge, but her ult is just a magnified version of her primary and secondary fires [the simplest forms of healing and damage] combined. This puts Moira in an incredibly unstable and meta dependent situation.