[special-K]

I think they are very well loved and just as popular when you account for their lower production numbers. All you guys with Suburbans, don't you get people coming up and commenting, even saying "I'd like to find one of those to buy"? I don't think they are hard to sell or get plenty good money for, either. Maybe you are going by the extreme end of pickups that are ridiculously over the top. Or the ones that have had everything but the sheet metal altered or replaced and transformed into a completely different vehicle.
People do way more custom work to the pickups and the prices are based on what was done. This whole popularity in 67-72 GM Trucks comes from monkey see monkey do. I've been into these long enough to have been one of the few who (in relative sense) who were into them wondering why others weren't loving them the same. You were "That 67-72 guy" in your area if you were into them, and we found each other to talk about them and get the lowdown on finding parts, etc.
The monkey see thing is not an insult. It describes how it works. Your neighbor, guy in your town, friend, or other gets one and it makes you consider one. Maybe you already wanted one but weren't sure it was a sensible thing to do. Now that is answered when the guy at the office buys one. Then came the internet and expanded TV programming with shows featuring them, as well as the magazines more and more. Each time one of these shows or magazines featured them, we'd see a noticeable amount of new members joining and asking questions.
There are more pickups than Suburbans and K/5s, so naturally all the factors are decreased with them. You could say the same thing about C/30 cab & chassis trucks and long bed Stepsides and campers. I have watched them increase in popularity, with LWB Stepsides going from "Redheaded Stepchild" to "Those things are just too cool". But the fact is, there are many less to be had and not as many, still, appreciate them. Most truck guys started out as car guys and it's the body lines of the Fleetside that made them even consider going with trucks.
The truck thing in general gained popularity, first, with 4wds. There are not too many 4wd Suburbans to be had. And they are at the heavy bulky end. But anywhere you see a Suburban mixed in with pickups, 2wd or 4wd, you see them very well accepted and loved. The fact is, unless you are an investor, none of this matters. The only thing that matters is you love yours. AND... if you decide to sell you can get damn good money for it if it's nice. At least from my perspective that is true.

The only other factor I'd add is a Suburban is harder to restore. If for no other reason, now that body panels at least are made, there is way more to them and the Suburban-only parts are harder to find.