It's a combination of things, really.
I currently have two nice older pins listed for sale here on pinside. A Haunted House and a Mystic. I have had some interest, but no trigger pullers. If the prices were dropped lower, I feel they would sell quicker, but I think they are currently priced okay. So it's a waiting game as a seller.
One thing I do notice (and have for years) is many people ask way too much for their games. Some of it hinges on either being too greedy, or investing too much into a restore, mods or just fixing it, which should not dictate the current market value. Then there are some that just don't really care what they list it for. If it sells, great. If not, great. It's a game to them, just like the hobby is. Many many people have pinball machines because they like them, not because they can play them well.
A lot of new games have been promised and pushed out, and the new stuff is really good stuff. If people can afford them, they want them. Some of the older games also have a cult like following in comparison to other pins, and naturally they sell easier. A game like Whirlwind or Elvira, are quick sells in general. So many people grew up or played that game somewhere 20 years ago, so it tugs at them as a buyer. Of course, pricing is everything though.
The bottom line is, they made hundreds of thousands of pinball machines, and they continue to do so. There is always something to buy, so I believe some have figured out how to be patient.