The Wagner drives the set....
The Plank, The Magie, The Doyle....The "Famous Errors" like the Doyle And Magie..
Those Big cards alone give the set recognition........
The mystique of how they were printed/produced/print groups, sheet arrangements...
The many mysteries like Brown OM, Brown lenox, Cobb Back...
The affordability (if you exclude condition and the big cards)....
The cool print scraps/errors/ and factory misprints almost create one of a kind cards within the set, or an "alternative" T206 set to be had, my favorite
...
The endless combos 5,500 + combo's????
The availability, surplus, yet rarities,
Cool ass tough shortprinted backs.....
when you think you figured it out, the monster tricks you and you learn different things about the set almost daily...
The beautiful lithography not reproduced in many sets...
The inclusion of many HOFERS...
Iconic Poses...
Many ways to collect the set(sub sets, teams, poses, hofers, backs)...
Liquidity....
Great investments...
There are a TON of other reasons, that I will let other Chime in
