The difference is products using Poser Shading nodes as opposed to Texture files. Daz Studio cannot read Mt5 or MT6 files. That's where suppose issues come in.

Also Daz Studio came onto the scene after Poser. Another issue is that Daz does not have access to the Poser SDK.

In the end, it is the choice of the merchant is they want to do the extra work needed to make the product D|S compatible. It is not as easy as it sounds.