If I had to guess it is the fact that it is a currency that can be manipulated outside what affects gameplay time. I think it’s clearer as an example. A very simple example is that enhancement prisms (at least in my opinion) are the least sustainable resource in the game without heavy gameplay or rip off currency exchange at rahool. Therefore, in order to have the materials you need to upgrade armor it requires players to play for large amounts of time in order to have any of the materials they need to try out new armor sets, making builds and such. Veteran players can sorta circumvent the system by converting stockpiled masterwork cores into prisms but at a rip off price meaning that it is accomplishing two of Bungies goals at once. Slowing down build freedom by requiring large amounts of play time in order to upgrade anything, and 2nd, draining resources from older players without the backlash of saying they are “retiring” masterwork cores or something. By rewarding bright dust, which is typically only able to be used to buy cosmetics that are older and have already made most of the real money people were going to spend on it they are giving a resource that has little to no effect on gameplay time. Most people that buy cosmetics with silver are going to buy them when they come out in the store anyway, not wait to buy them with bright dust. So it keeps players interested in a valuable resource (for those who like cosmetics anyway) while not speeding up acquisition of materials that have an effect on gameplay aspects or that have much impact on their sales. TLDR: Bright dust allows bungie to provide a wanted resource that does not cut into gameplay time, or silver sales by a meaningful margin. It also means that anything developed for eververse has even more justification for development time than if it was still silver exclusive. They get to “double dip” on eververse items by selling them once for silver and then once for bright dust.