The main design concept behind the pouch was having the ability to hang the pouch from your tripod while shooting - at the time this was a unique idea and it made using the filters while on a shoot very convenient.
The resin filters are quite delicate, and they need to be stored flat and with even pressure exerted across the surface if stored for any extended period of time (they can bend or warp if not).
Glass filters (especially our stoppers) are brittle and can also be easily broken in storage (which is why we supply them in a tin).
The ideal way to store any quantity of both types of filter is together, in a flat block where they protect each other, like layers of laminated glass - once your filters are in the closed Field pouch, they are extremely hard to damage, even if you try (which of course, I have).
Introducing a hard, unevenly shaped item like the filter holder or adaptor rings into the pouch would mean the addition of larger compartments and a lot more padding to protect the filters from damage, easily doubling or possibly trebling the size of the pouch - in all likelihood making the pouch too big to fit inside your camera bag (we made sure the field pouch would stand on edge inside the most popular camera bag brands when designing)
The filter holder and adaptor rings are very tough - virtually indestructible in normal use and they don't really need protecting in a pouch like the filters do. The holder and rings can be stuffed into any pocket of a camera bag or rucksack with no chance of them coming to harm, and of course when shooting, the holder and rings are in use, on the lens and do not need to be hung on the tripod for quick selection.
I hope this helps explain our thinking when designing the Field Pouch - you cannot please everyone when inventing products for mass market but we believe the filter-only pouch is the most sensible compromise for a pouch for use when actually shooting images - maximum capacity in a minimum size, with the filters protected and conveniently hung on the tripod for quick access.
If you want to keep your filter kit together in one place when not in use, well that's a different matter.