As Boomers would say: Mayfly Attention Span.

Starcraft is EXQUISITELY optimized for e-gaming and streaming. That means short games and relatively small maps and relatively small numbers of units and "snowballing". Small advantages from a micro battle or macro organization multiply quickly into an inexorable avalanche. It's why Blizzard has to CONSTANTLY jiggle things to bump the meta around.

Supreme Commander, by contrast, tended toward larger maps. Your Tier 1 unit rarely could find the opponent before they were already laying out Tier 2 buildings. Battles had ebb and flow and RECLAIMING after a battle occurred meant that if you survived a battle, you now had material to rebuild and possibly more than the person who launched the attack. The game was more homeostatic than avalanche. But that doesn't tend to make good e-gaming or streaming fodder for ADHD teenagers.

And the interface differences are MASSIVE. I could go on for HOURS about why I LOATHE the Starcraft interface and love the Supreme Commander: Forged Alliance one (and similarly Beyond All Reason). But, that's a personal preference thing.