According to the current meta, the support role is someone who can peel for his team while still being impactfull on the enemy team.
Commonly played Supports on LCS:
Allistar : Why? Huge CC kit, can easily act has an engage or disengage and can also act has a huge Frontline for your team while simultaneously peeling
Morgana : Completely negates Pick comps, can allow your team to perform a pick, due to her Ult you can stop the enemy frontline from diving your backline
Braum : Same as Allistar pretty much but provides a Wall that blocks pretty much everything thrown at it, used properly can turn around a teamfight
Where does Maokai fit in all of this? Yes he has a good kit with sustain and Poke, but he falls of pretty quick if you don't itemise tank items, and in a teamfight he won't be diving the enemy backline probably, not that it's a bad pick, just not viable, if you get paired vs 2 ranged you won't do much cuz you can't freely engage or go for trades, while it might be a solid pick since it can provide the required by meta engage/disengage , there are more solid picks for support that better complemente this, for instance if you pair the Maokai vs Ez and Morg he is gonna be useless in the laning phase since the morgana shield will deny him of any engage opportunity and Ez can just E away