The last time we got a NEW mayor healer champ was Nami :/ bard's shrines are very MEH and quite frankly who actually plays bard and rakan's q is also not that strong plus its more like the heal was added for flavor, it's not his bread and butter to heal.
Soraka doesn't feel good to play as she is literally just a walking summoner heal spell on goat legs that heals good but hurts herself.
And i'm not saying make a new soraka >-> Look at Nami, she is a healer but is not 100% based around it like soraka.
Sona is a nice healer too, but she has various problems with her as it is and she is more of a late game champ ESPECIALLY a late game healer.
Riot cannot deny Sona's popularity as a champion and how healers are a popular play style in gaming.