Soraka is passive, but less passive. I'd presume a lot wouldn't want to gank the lane unless it was just open and begging for it still, but she can follow up CC or initiate with a silencing root, then a slow, and probably exhaust while dealing more damage than a Janna would. Janna's Q makes her more unreliable offensively since it's telegraphed and slow.
However, I still don't see many more ganks when I'm playing Soraka than Janna. The jungler has to consider where their time would be most effective, and a passive, self-sustaining lane that's not being dove or something isn't the highest priority.