Because PS is not considered a Polaris Business flight. Polaris Business Class is only for international long haul flights. PS is simply "Business" Class." Note that you do get access to the United Club. While not a nice as the Polaris Lounge, it beats sitting at the gate.
On a more broad subject that is OT but has been mentioned before and was just mentioned in the previous post, A lot of people ask why [this] and [that] don't get access to the Polaris Lounge. Keep in mind that if UA allowed access to the Polaris Lounge for all of these odds and ends (PS, Hawaii, 1K, GS, any flight with Polaris Seats, 2 credit card passes per year, etc.), the Polaris Lounge would not be the treat that it is, but would be more like the United Club. It would be crowded, show more wear and tear, and as costs rise due to more use, cuts would be more financially benefitting to UA. The idea of the Polaris Lounge was to be exclusive and not for everyone. -- If everyone is special, than no one is special.