The future of rare backs/variations...

[steve B]

I think Pete's right, the other sets won't really be examined as closely, and the varieties won't bring the sort of premiums some of the T206 varieties bring.

I think there are a few reasons.
T206 is massively popular. Partly because of the Wagner, partly because it's so available. And being a big set it can be collected many ways. subsets, all one back, teams, just Hofers.
It also has the couple things going for it that make it fascinating over a longer stretch. Even the 520 or 518 set has the appearance of being "easy" And with a decent budget it is. But it can also be challenging, especially on a budget or if you're somewhat picky about any aspect of the grade. Some are really tough with good registration and centering. Some are tough in anything over vg-ex.
Any of the subsets are challenging, but not impossible.

And that goes a long way for popularity.

I also really like the less popular sets. But I don't have much beyond a type card for most of them. Trying to collect them as sets I think would be a real challenge. Especially the Venezuelan cards. WWG seems available, but not as readily available as T206 or Goudeys.

There also hasn't been the effort to figure out the varieties in those sets. And that's possibly because those sets and T206 were originally looked at differently. T206 has always had varieties that were sort of the stars of a collection. Magie, the STL Demmitt and O'Hara, three tough cards that are variations.
None of the other sets really had any "big" variations. Yes, Goudey has a few diferent Ruths and Gehrigs, but they're different numbers so clearly not variations. And there isn't a huge difference in difficulty.
So for lack of a better term, looking for varieties was and is part of the culture of collecting T206, but isn't for other sets.

Some sets are simply too tough. If a needed card comes up once in maybe 5-10 years, most people become bored and move on to other things.
Oddly that's not always a problem in other hobbies. Some collectors of stamps might only ever have one chance certain items in their life. The stuff I specialize in right now has stuff I know I'll never own. Either because of cost or because it's both expensive and owned by someone about my age. The next time some of it will be sold might be 20 years from now.

Steve B