Miles have said a lot about this on Twitter before, but the general idea is that SI get feedback from Steam regarding what types of specs their users actually use. And they base a lot on that, as far as I can tell. You can even take a look around the bugs forum, especially technical issues, and see that a lot of users are having issues because their specs are BELOW the recommended. So if SI were to stop supporting anything, they would eliminate some of their user base.