Because you might want to change technique mid-fight. I know I decided to change my default after trying out moves i didn't use before. So now I can access my full repertoire without dealing with menus.
~~And, restock? Unless it's the free supplies, that's a skill issue, dawg.~~ And restocking takes time that you'll not always going to want to take. I hating having to wait for the animations to finish and only have to do it when I've prepared drastically wrong.