Different sleds for different riding styles and body types. All sleds are more "rider forward" than they were 10 years ago. 10 years ago, sleds were made for sit down riding with occassional standing for tricky sections. Now, we want to ride standing up the majority of the day....
The cat rider position is just far enough back that is has a noticeably longer hood than the others, which make it look "older" in style...which, of course, it is so that makes sense. If you don't think the next Cat will look more like an XP and less like a 1M....who are you kidding?
That said, the more upright style in itself does not make the sled more tippy, easier to sidehill or anything like that. For example, an Edge RMK with the swaybar removed and a narrowed front end was as tippy as anything. The more upright style of sled makes for more clearance for longer-travel suspensions, and everything is brought closer and closer to the (stand up) rider. If the M sled was "upright enough," why does it keep getting higher seats and higher bars, right?