I was perusing the archive of the old blue boards tonight (thank you @Michiyo ), when I came across an old post of mine quoting some game statistics Paragon Studios released in April 2011. Unfortunately, the game statistics Paragon Studios released appear to be lost in the sands of time. However, according to my post from thirteen years ago, City of Heroes had a roughly 60% hero/40% villain population, per the statistics Paragon Studios released. Granted, I'm biased, but I think I'm a pretty reliable second hand source.
According to a Reddit AMA, when the game sunset a year and a half later, the game's population was roughly 80% hero/20% villain. That's a pretty big drop in villain population. Why would that be? That's easy. Side switching (which became available a few months before the statistics were released), and the ability to create all ATs blue side several months after. So it's pretty clear that the red side ATs were a big part of the draw that red side once had. I'm not sure, what, if anything, can be done to make up for that. That was a mechanical game advantage or draw that red side had, not a flavor one.