Because it's a setting that's been beaten to death by games companies. Remember 4 or 5 years ago when 95% of shooters released were set in WWII? It was a running joke in the games industry. There are just less WWII gaming nerds than sci-fi or fantasy gaming nerds.
I sitll think a well designed game would succeed. COH2 has no one huge flaw, but there are many little ones that add up to a bad feel for both casual players, historical players, competitive players, players looking for a fair fight.. etc.
I think the original campaign didn't help them either. The original COH campaign was really good, especially for its time. Playing a mission or two as a demo made someone thing "this is SOO cool! I have to get it". That draws in the players, and if the game is good it draws them into multiplayer in the search for more content. The active player base meant you find other noobs with whom to get your feet wet either in 1v1 or in lobbies. Smurfs were a problem for noobs, but luckily they ranked up past you pretty quickly.
For COH2 Relic chose a faction that was not particularly popular (Soviets) and brutalized the history. That can knock a good # of people out of their "suspension of disbelief" which is a big no-no in games as well as movies.