I think some of it is a restriction with how the game handles buying items with currency, versus trading them for tokens.

Shop tabs where you buy items only take one currency at a time, but a trading tab (like the Wondrous Sundries where you trade in your set of raid tokens) can take a broad mix of items.

So firstly it would cause an issue because you can't trade three different currency coins into the one purchase any more.

Also, unless they greatly simplified the number of currencies, every one might need its own tab in the shop. There'd be a page for Gordian headgear (lens currency), then body gear (shaft currency), and so on. More categories needed per raid, and maybe separate tabs for every item-converted-to-currency in the Sundries trade section.

So, assuming I've guessed it right at least, it's probably not as simple as it seems like it should be.