It’s the availability of modulars from Lego, that’s an effect on secondary pricing.
I currently have a BB, PR, GE, DO and a PC on my desk. It’s all I have room for.
Lego currently have Asquare, PR, DO, BB, PC available, not to mention Ninjago city and the fishing shop. People just don’t need to buy a retired modular to have a decent set up. And there have gotten to be so many, it’s tough to collect them all. Even if you have the budget, space becomes an issue. A couple of years ago, before BB, DO and Asquare, (and even the simpsons) choice was a little more limited and picking up a retired set or two may have been more desirable.
PC I’m not sure about, for the casual or limited budget, there are just better options available at retail.