^Your argument would be valid if Bioware have actually said they only decided to up Cerberus's game *after* they made ME1. They were always going to be players, just not as major as they turned out. I'd provide a link, but I can't be bothered.
Doesn't matter. EVERY aspect of the Mass Effect universe was more thoroughly developed in ME2, including Cerberus. There were, in fact, quite a few things that had only niche roles in ME1 that came back in a huge way in the second and third games. Notably, Nassana Dantius, the Terminus Mercs, the entirety of Krogan civilization (such as it is). And going from vague references to "pirates in the terminus systems" to "The Eclipse, the Blood Pack, the Blue Suns... also, don't fuck with Aria" was a much bigger change than finding out where Cerberus kept their more interesting toys.
At some point you have to take into account that Mass Effect 1 really wasn't THAT sophisticated of a game to make a fair comparison to the sequels.
Guess which ones really caught people's attention.
Quarian marksman was mine. I immediately thought "somebody read my mind!" I've been using a quarian character as a dedicated sniper for months; they seem faster, and their camouflage lasts longer, a combination of traits that is IDEAL for a "shoot and scoot" plink-artist like myself.