Short answer: Money.
Part 121 operators, i.e. airlines, are interested in profit and running an airline is a low margin business. Aircraft are very big capital expenses. They cost a lot every day in interest and depreciation whether they are flying or not. But they only generate revenue when they are flying, so airlines try real hard to maximize the days the aircraft is in service. About 20 years ago, an airline told us that taking a 737 out of service for one day cost them $65k in lost revenue. On top of that, crews have to be trained to fly LPV approaches. That adds training costs.
Adding LPV to an existing aircraft requires an STC. So modifying aircraft to add it takes time and money. And sometimes the time out of service costs more than the actual labor and material to do the mods. Before an airline will make the mods, they will do a benefit analysis to see how long it will take to earn back the cost of the mods.
The issue with something like LPV capability is that it is only beneficial to the airline is if they have to have it to provide service to the airport. If all the airports they fly to have ILS approaches (and almost all airports in the US with scheduled service have at least one ILS), adding LPV doesn't add any benefit, so adding it is a waste of money. If they want to add service to an airport that only has LPV, then it has value. The next question is; how many aircraft suitably equipped are needed and how many crews need training? In the end, it makes business sense or it doesn't.
I suspect that most of the aircraft that are or will be equipped in the future are coming equipped from the factory. The incremental cost is small in production. Then the impact is training and scheduling.