The core reason? If you stuck a gun to my head and asked me "why Supcom"?
Because its Supreme Commander. I grew up on OTA. Its first RTS I truly played, I forgot why I migrated to Supreme Commander and later Forged Alliance (heck for the longest time I think my memories of FA were actually Supreme Commander Vanilla). I've tried other RTS, be it Warcraft III, AoE and its clones, Starcaft on N64 (that was an experience). But I return to Supreme Commander, why do I return?
Because Supreme Commander is unlimited, in what other universe set in an RTS that the game, is such YOU matter. Not a Heroic Character with Name, but you. You are the ACU, you are in the cockpit. And the scale of the universe, is that YOU don't matter. Its a thousand year world, fought on practically infinite worlds, across time and space. Nothing YOU do matters, so YOU be anything you want to be. You can do anything, be anyone and the skies the limit. And their the core game design, its inituitive by beyond inituitive its seemless.
One of the flaws in Starcraft or AoE is "Build More Farms/Build Additional Farms" or how non flow-eco gameplay interacts with the gameplay. If I need 100 mass to buy something outright waiting for 100 mass 'slows' down the gameplay. A flow eco enviroment like in Supcom means you are never needing "100" mass you only need a mass income = to what your consuming and you stall, game doesn't stall. I can a unit while stalling at 90% it just takes 10x as long. Its only downside really is the tech system in FA Vanilla is inorganic. I tech up a factory but it doesn't 'mean' anything. A mod like EngiMod however solves that beautifully. As now Support facts make the transition more seemless. Now I have issues with EngiMod (not balance onces philiosphical ones to be clear). People like to say Starcraft is constant 'action' that true but that because of how the game itself works. Due to how I described backqueing is bad as you essentially spend resources without spending them. In Supcom the game itself core lends itself to things constantly happening.
And what Sprouto mentioned did you know Warcraft III, has a hard coded 4 faction limit? Legit its impossible to add more factions to the game w/o removing a base game faction. I have a clear bias here, but think about what that implies for a second about the game code.