Well for several reasons. For example the Xbox One has only been available for three years in some countries like mine, The Netherlands and 4 in the other countries. That's not a whole lot. Plus if every game from now on would only work on Scorpio that would mean Xbox One owners have nothing left to play. Maybe it's not a stab in the back, that's too extreme but it would be asking for trouble.
I bet plenty of people would be extremely pissed off and not bother with Xbox anymore. At least one friend of mine really isn't planning on buying a new console already and he's contend with his X1.
Microsoft have stated billions time it's not next gen and you know that.
Yet, here you are...