It's more or less an issue with Janna's kit as a whole. She has two sets of disengage, an area heal, a shield that also grants AD, and a point and click slow.
Basically, she has too much that a marksman wants. Lulu can give attack speed, movement speed, the on hit bonus of pix plus a shield, and an ultimate that increases health. She also has a skill shot slow. Compared to Janna, she lacks disengage, and the multi-use functions of Lulu's kit ends up with no ideal scenarios compared to Janna's "never let them touch the marksmen" strategy. Not to mention that attack damage is more useful than attack speed to a marksman, while also protecting them.
Janna is honestly a woefully outdated champion that should at some point receive an update just to make it more interactive between herself and her enemies; right now you have no way of denying Janna from protecting someone, and no real way to kill a Janna that's playing from far away (excluding a good assassin, but in a scenario where it isn't just Janna and the marksman, that's fairly unrealistic (not to mention that Janna will just force the assassin back and they will likely die to the rest of Janna's team.)
That's just my two cents on the matter. It's not so much that the other enchanters are bad, it's that Janna provides too much.
