It's because they have never put any effort into balancing or updating her. As long as she remained obscure they just ignored her.
Originally in beta she was more of a bruiser. Her invuln shield was a normal ability and self only. Her ranged mode was her ult and had a long duration/long cooldown. So she was pretty much a bruiser. I think her heal could target allies though.
I don't know why they decided to make her into a weird hybrid of support/ADC/bruiser but they did that pretty early on and just left her like that this whole time. They never even bothered with her base stats so she still has her old bruiser stats.
Apparently, they just don't give a fuck about her. Her kit has always been an incoherent mess. There has never been a time when it was up to the standards of the game even before when the standards were a lot lower. She has needed a complete remake since the day this game was released.