If you are choosing most convenient flights - by routes and schedule - your upgrades compete with affluent customers who buy convenience without considering the price premium.
Choosing the opposite flights greatly enhances the success rate of upgrades.

For example, there is a SFO-SEA flight that leaves at 5:50 in the morning.
there is no way first class seats to be fully sold on that flight.
I took that flight because I had a creative combination of arrival from Europe at 11pm with reward ticket from SFO.
Lounges at closure and before boarding, with a stroll around SF in a Hertz car.
SFO-SEA was upgraded 5 days before the flight