I think it's because for most collectors, the OT is more nostalgic because many collectors are likely in the age range where those are the SW films from their youth. So between the two, they'd be more apt to splurge on the OT than the ST.
I grew up with the OT, but my attempt is to have a healthy balance of both. I pretty much have all the essential characters from ANH and for many of those characters, that will be the only OT representation of them I'll get. Leia, Han, Chewie and most likely Vader all fall into this category. For ESB and RotJ, it's all just select characters: DX07 Luke, Yoda, SSC RG Luke, SSC carbonite Solo, RotJ Luke and maybe the Emperor. With the ST, the majority of my ST collection will all be from TFA and I won't get duplicates of those characters. Finn and more than likely Kylo Ren, for example. I'll likely just be getting another Rey and Luke from Episode IX for the ST.
So basically, my main SW collection is based on the first film in each trilogy, with a few interspersed figures from other films in the trilogies.
If HT ever delved into the PT, RotS Anakin and Obi-Wan are all I'd be interested in.