Most are like me who want to play online "for fun" and dont want to watch hours of replays or read hundreds of Strategy guides to be able to play against a stacked team of smurfs (Smurfs is far to good a word for these guys). I love RTS and so do my friends but we gave up at online long ago.
I do not play all that often and still find the AI fairly challenging on unfair.
And team stacking is not just an RTS problem as it was rampant in Left 4 Dead as well. I maybe twice seen an actual clan vs clan. It was always clan vs n00bs with the clanners laughin or insulting the n00bs the whole game. When I finally got good they always invited me on there team only because they did not want to die or dont like a challange. I told them I would rather play with the n00bs. At least they did not smurf.
Anyway the teamstacking by smurfs is very annoying and I totally agree its a big reason All RTS games have very few players.
I am a regular Battlefield 1942 player and every night there is 500 -1000 players online. The only smurfing I know of is admins, not for spying or team stacking, all players target admins as its like killing an officer lol (myself included). So don't blame the age of an obviously classic game.