Or because they are not as much similar?Because there's FAR more of them, that's why.
And, you know, RPG is also a big genre, if you don't want to see all the sub-genres. Space sims are also have sub-genres - space shooters or action space sims (X-Wing, Tee-fighter, SSZ), space flight simulators (Orbiter), elite-like space sims (X-series, Freelancer, all Elites obviously, etc) and sandbox space sims (which are not failing into elite-like space sims for some reason, like Terminus or I-War). And yes, MMO space sim is also a sub-genre.
Are SC and X:R in same sub-genre? We don't know. Theoretically both are elite-like space sims, but you can't say for sure until release.