My guess...because "Cash is king"
Sure, there is the debate that miles is a form of currency, but I believe UA wants to get some real $$ for it's upper cabins and limit the number of non-cash people on the plane. If you book 20 people bok xy, and then 10 of them upgrade to NF, then there could be 10 more xy seats open and then you would have more non-monitary people on the plane, lowering the total revenue.