Because the game's UI is bad, and this is one of the many issues. If you have two weapons of the same name, you can just hit your hotkey button twice and it will equip both. However, if they have different names, or even the same name but enchanted/improved, you have to manually use the item or favorites menu to select it. I tried DWing for a bit, but went back to Sword and Board after a while since having to constantly stop the action to use the menu to equip both weapons really hurt the pace of the gameplay. I love Skyrim, one of the best games I've ever played, but i swear Bethesda saved the UI for the last 2 days of programing and were drunk the entire time, it's the only explanation I could understand for allowing so many problems and inefficiencies into the UI.
In a 5 minute battle with a room full of enemies, I ended up having to hit up the menu just to switch weapons nearly a dozen times(between having to heal, switch to some spells(I'm a battlemage type), switch to a Sword and Shield), all of which are properly hotkeyed, but every time I wanted to go back to my main 2 weapons, I'd have to pause the action. As much as I respect Bethesda and the quality of their content, stuff like this is just an awful oversight and never should have made it into the game's release version. I really hope this is fixed with the next patch, or they release the construction kit soon so the modding community can fix for free what Bethesda gets paid not to
