If you could use both grenade types at any point, then they would have to add in an additional UI element for the other grenade type.A possible explanation is that they had more nades planned.
But I bet some UX fucktard went like "can't have both binds on similar buttons on controllers which means they can't do similar actions".
I'm guessing a controller is exactly why. Swap grenades button is probably one of d-pad buttons, which are conventionally used for switching things. Even on Nightmare I still wasn't using frag grenades because of this. Not that I ever felt I needed them, there's already enough shit to juggle.
That sounds dumb though. games like COD have had smoke and frag grenades on separate controller buttons for ages, and unlike DOOM also have aiming down sight.
IMO I think it would have been better if both grenade types shared the same resource (give you a reason not to cheese with the Ice Bomb), though the UI itself would have been much better if most of the equipment counters were moved around the center of the screen rather than your periphery.