What kind of dumbass company is going to spend money porting a new version of an OS to an old platform, with no payday for doing so?
Well, OK there must be a payday. Perhaps they see the fact that you can put the latest iOS onto a 4s as a selling point. i.e. if you splashed the cash for one in 2011 you would feel better knowing that, theoretically, you could still have the latest OS four years later even if unreality you replace it after two years.