I'd imagine it's probably because most users got DAZ figures long before they got P6 or higher (P4 days myself) and so had lots of content already which they continue to use, and when figures were updated it was a natural migration.
Also the fact that DAZ figures have much more items available than the native Poser figures means that's what people tend to see in the galleries and so you get the 'hmm, they used DAZ figures to do that pic, that means I need to use them too' mentality. (Similar to other pusuits, ie, photography where amateurs see a great pic was taken with a such'n'such camera/lens combo and so think they need the same to take that type of photo, whereas we all know it's not always the equipment, it's the user.)