As for the limit, it never was really set that way, other than to see what kind of "heat" can be generated. And boy was it, hence why the servers and key servers were loaded up for the long haul.
As for why, It's because MS is truly listening to the general public. It's no secret that the past the general public's voice was seriously forsaken to the likes of us MSDN & technet folk. What you had with Vista was more of a technical piece of OS, rather than simplicity. Again, as I mentioned before, please send in all your feedback to MS, there listening this time.