Why isn't Sona played more?

Basically the title. Why isn't {{champion:37}} played more in this {{item:3504}} Support Meta? You can spam her heal which also grants a small shield to all players in range. It basically applies {{item:3504}} passive constantly right? So why isn't {{champion:37}} a top 5 support pick right now? Am i missing something? Edit: Should have mentioned i've been out of the loop concerning LoL for awhile now. I've played most aram games just to kill time but haven't followed the game/patches or even pro play. Since worlds came on i've been reborn lol and my interest for the game is coming back and it was something i noticed while watching worlds and wondered what was wrong with Sona while janna lulu karma rakan and taric were being played alot during pro play but was wondering about sona. Thank you for the detailed responses they are a big help.
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