Because that would be unbalanced. Do you know of One-Punch Man? The short of it is a guy decides he wants to become a hero, in a universe where there are heroes with superpowers and monsters that they fight to protect other people. He physically trains to become powerful to the point where one day he's suddenly arguably the strongest, fastest being on the planet and can utterly destroy most monsters in a single punch. This has led him to realizing no enemy is ever a challenge for him and he is always bored and disappointed with how easy beating an opponent is. If we could equip exotics in every slot at the same time, we would inevitably all end up with a similar outcome: boredom and a sense of no purpose to playing because everything is too easy. If we were to be able to equip multiple exotics, then the paradigm of how enemies are challenging would also need to change.