People like to join rooms that have people in - they tend to go more for rooms with more people in.
So when they were listed out in full, people might pop in if they saw someone there *anyway*. Or if most of the public servers was full, they could see this long list of empty servers to use.
Now it's all hidden away and doing what it's "meant" for - it's not attracting ad hoc use and, really, there's not that much planned activity.
(I keep thinking "I really should get around to checking in with @insert_user_here@ to see when we can have a jam"... and just don't do it. Sorting out using a private server is one more step
)