Ahhh, I wonder whatever happened to SupCom? That coulda gone on as a full series.
SupCom2 happened.
As far as I know, Supcom 2 trumped the original in sales by a large margin. It also received very positive reviews in general, including being labeled the co-op game of the year by several notable gaming sites.
It seems like there's this cult fanbase that pretends Supcom 1 was some successful or revolutionary title for the genre, and I hate to break it to those people but it wasn't. It really didn't do much, 20 years later, that Total Annihilation hadn't already accomplished. The resource requirements were absolutely insane, so that 75% of the potential playerbase couldn't even use it, and the game itself was kind of an unorganized mess, the story was lackluster, the multiplayer was buggy and laggy. It just wasn't a great game, even with the standalone FA expansion. It didn't tread anywhere new or do anything particularly well.
Supcom 2 wasn't all that revolutionary either, but 5 years later it had graphics which trumped the original and were about 75% less resource intensive. The "research tree" was also a nice touch and had never been attempted in the ground-based RTS genre, at least as a resource in a fast-paced setting (4Xes are completely different). It's true, the game left a lot to be desired, and deviated further from the original TA even more than the first game did, but I overall I thought it was pretty solid.
In other words, don't blame SupCom 2 for the failure of the series, if the first game was any good they wouldn't have even needed a sequel. The first game was a disaster, much more so than the second.