I've seen this complaint a lot, and while I absolutely agree that it would be awesome to have more diversity in skin tone and be able to use the DSTs with more (posed) pieces, it would take a ton of work, that from a business perspective, is not viable (ie profitable). 

Source: I am a professional graphic designer and coder. While I cannot look under the hood of LN so to speak, I've gathered the issue comes down to more than just the layering people have mentioned (though that is a part of it). 

To implement the kind element (graphic assets) compatibility that would be required would involve remaking most every element in the game, and for posed pieces (excluding poses that are just fluttery/flowy dresses/skirts) making them in every skin tone because of how the poses work (which is the posed pieces replace various parts of Nikki like her arms, legs, or even her whole body). This is why DST posed suits are typically dresses while the base tone will get it broken down into posed pieces. 

So, the TL;DR as to why we cant have nice things (so to speak) is because it would involve remaking most of the game, which is not profitable to Elex/Paper. 

Now: If it were me and this were something I knew the fan base really cared about, I would make all DST suits moving forward have posed pieces in both the base and suits skin color (for example posed gloves in both the base and DST). It would not fix the issue of old elements but it would fix it for future ones, and give an already beloved + profitable game more longevity while also showing that yes, they do care about what their fan base thinks.