Playerbase is too small.. The multiplayer part needs to be free to play in my opinion.
The small player base has nothing to do with the free-to-play feature. Starcraft 2 is not free to play and has a huge player base - they need to be devided in leagues. The reason for which COH has so small playerbase is
1. the difficulty of the game and the months / years you need to practice to understand it properly and be realy good at.
2. the balance problems. Instead testing enough before release they test it on the player base and make constant adjustments. This streches players' nerves to maximum. It's much more complex and harder to balance than SC2 though.
3. people who like glowing lasers starships and drooling monsters are more than those who loves shermans and panthers.
4. coh2 has no proper ranking / elo system. At least COH1 had it.
5. one of the worst optimized games. You need to have a supercomputer to run it and you could still have some lagg, conection, driver, etc etc etc.... problems.
6. it's not cheat proof. By the way, if it will become free to play, many good serious players will abandon the game because of cheaters' invasion. This kind of people will use cheats just to shine a few months then they will live the game to cheat in Counter strike or another shooter or NFS game.
7. bugs and excessive RNG are killing the game.
All these are making COH2 what it is, a game ignored by many, but truly loved by few fans.
I think that Relic will start to solve this points one-by-one (except 3 of course), the player base will constantly increase.