It's financially impossible to collect it all these days. Many people focused on a favorite player or team, perhaps because of the interest level, perhaps due to financial constraints. Time became a huge factor and still is in a way. You can spend hours on the different sites scanning lists for players or cards you need. Inventory changes daily on many of these sites and if you snooze, you lose. After a while, you focus your searches to what you really want.
The error/promo/prototype craze started right about when the insert/parallel craze did for the most part. 1989 UD and their Joyner variations and the Dale Murphy reverse negative. Heck, you could even go back to include the early Fleer/Donruss errors and such. Remember 1981 Fleer Craig Nettles! We can't forget Bill Ripken either. Topps was making errors early on as well. Those are still collected, but not in the way they once were. Supply caught up with demand in many cases. The real tough errors are still in demand and priced accordingly. I have no interest in a 1982 Fleer John Littlefield reverse negative card, but I'm sure many still are and the prices reflect this.
Many of the promos were issued in much larger quantities that you'd expect for a promo, hence the low value of those.
As much as it pains many collectors who overpaid for things when they were hot, the market will settle in the end the real value and interest will shine through! 93 Refractors for example are still popular, even though they took major hits from their peak values, but they will never be a cheap as other inserts/parallels of their time.
