The story with Lulu is pretty depressing. Let me tell the story with patch notes:
V3.15
Lulu's defensive capabilities were a little low for a traditional support, so we sent her to the gym to get buff. Like most supports, we're keeping an eye on Lulu as the preseason progresses.
V4.5
Ultimately we had to hit some of Lulu's high offensive power, but we're also committed to ensuring this doesn't have a huge impact on support Lulu (who actually comes with tradeoffs like no guaranteed farm or gold generation), so we'll look at ways to help support Lulu if she comes out too weak because of these changes.
V4.13
We initially thought solo lane Lulu was a unique addition to the game, so we wanted to let her stay a viable pick in both solo and duo lanes. That said, top lane Lulu is currently shutting down a lot of diversity in competitive play and, because Lulu's solo lane and support power are tied so closely together, we had to go for changes that reduce her oppressive strengths first. As an aside, we know this isn't a great situation where we keep indirectly reducing support Lulu's power to maintain her viability as a mage, so this is something we'll have to really consider for the future.
So, uh, how's that "consider for the future" thing going, Riot? crickets
tl;dr - Solo lane Lulu isn't healthy, particularly against melee champions. Riot was hoping to nerf her in ways that wouldn't affect support Lulu, but just like always, that turns out to be wishful thinking. Support Lulu won't be making a comeback until they can figure out a way to make solo Lulu not such a bully.