This has been an interesting discussion, and I think both sides raise valid points.
I think it's partly because being "heroic" just feels better than being "villainous" and players will naturally be drawn to blueside. That's certainly a factor.
However, the fact that CoH was first, had developed a playerbase for at least two years, and started out with vastly more missions, zones, and quests than CoV did definitely plays into it. While it is true that the CoH developers learned from a lot of their mistakes and did a lot to fix them when they launched CoV (less pointless zones, more streamlined missions/quests, etc.) the result actually played into CoH's favor.
I think had CoH/CoV been both launched at the same time, and had both sides started out with an equal amount of gameplay, the player base wouldn't be so tilted in favor of blueside. However, redside was doomed all the way back in 2004 simply by virtue of not being present when CoH launched.
If I had the money, the time, the coding skills, and the writing skills (spoiler: I don't have any of these things), I'd take the CoH assets and attempt to create a "mirror" game that reimagines CoH in a way that provides more balance to both sides. One way I'd start would be to take many of the "Hazard" zones in CoH and turn them into co-op zones, and streamline the CoH zones (ditching one of Steel Canyon / Skyway City, moving all of the ditched zone's assets over to the remaining zone, and having one blueside Lv10-15 zone, for example). But then that's a completely new game.