It seems that Polaris just "petered out". United got fifteen 773's new and they were so configured. But they are in no rush to reconfigure any existing 763's or 772's, save reportedly one each.
Not sure where you're getting this idea from, but 11 767s have already been converted to Polaris, with two more currently in mod. In fact, there is only one 767 with "old UA" business class left, and it will go in for mods pretty soon. After that, three 767s that UA just bought from Hawaiian will go in for mods, along with, reportedly, some of the 767s that were initially modified to Continental-style business class shortly after the merger.
On the 777 front, they have 17 777-300ERs in service, with an 18th on the way soon. One has already been modified to include their new premium economy product, and the rest should supposedly get converted in short order. They also have five 777-200ERs with Polaris (and premium economy), with four more out of service for mods.
Boeing 787s with Polaris show up this fall, and the existing 787 fleet is supposedly going to go in for modifications by the end of next year.
Polaris did get off to a slow start, but it's moving along just fine now.