Another reason might well have been because too many player-characters on the screen would bog down some folks' computers and create lag at the least, if not making them go full-on slideshow. It happened to me during invasions if I got within view of where the main action was.
It's all those costume-piece options, I figure; the program has to track every choice each player has made. Games like GW1, with simpler costume creators, never did that to me.
That's among the reasons... one of the other ones, has more to do with optimal use of the hardware. CoH wasn't exactly built to run as efficiently as possible, from what I hear. One of the issues with the state of the code, from my understanding, was that there were a lot of layers built on top of other things, and kludging things together for graphics was among those things. So, while a game like GW may have simpler costumes, it ALSO has much better graphics in general (based on all the screenshots I've seen anyway).
Yeah, I purposefully played on the third most populous server (well, some sources say that, others say that was Infinity), but I loved Justice. It had just the right amount of folks to be active and easy to team on, but small enough that I didn't feel crowded or have unimaginable issues with names. It also had a very nice collection of server channels that nearly everyone used, so it was pretty easy to get stuff together.