It's still the successor of DOTA/WC3 and the "creator" of the genre which absorbed most of 90/00 RTS players. Despite the quality of the game, it has a bigger pool of people to fish from.
Even if CoH2 was the best RTS of all history, it would never draw too much players in comparison to other genres. The reality atm is that the majority of people prefer "fast food" like games, with immediate gratification/progression.
Pretty much this.
DoW2>CoH series in this regard with progression system(linear, grind based and gives only bisuals but army at 1lvl vs army at 50 level is over 9000 levels of awesome behind) and 15-20 min matches.
Most players are not patient, they expect quick games and instant gratification, something you will not find in RTS games.
Why do you think even Total war series tries to cater to that niche with TW:Arena?