Because she wasnt just used to beat Sett, Soraka top was first played by some main who took it all the way to masters and that´s from where the pro´s got it if i am not mistaken.
It worked because she due to her kit could still farm and shit on most melee toplaners and even if she lost her items are so dirt cheap that she could still end up getting them first and have more impact in the game overall.
When this was taken to the pro stage it became very evident that such a strategy was actually more effective than the traditional toplaners and it had much more impact to boot.
This meant that she alone kinda made 15+ other picks obsolete which very naturally hurts overall diversity and thus she got nerfed same as Sona if i am not mistaken who was doing the same thing with a similar story to it.
...
A champion starting to be played in a different role is not bad, what is bad however is when this comes at the expense of killing off a bunch of the champions already there.
We are looking for more diversity, not less.