Its been known since beta that you were only allowed one exotic armor and weapon at a time. And it's because the armor is OP to the max. Most of the exotic armor provides for one or more of a subclasses abilities or modifiers. Like the helmet if inner most light provides for both the death from above and the "sprinting makes you leap farther" ability (sorry I main defender). Then you still get to select other modifiers. Think if you had 4 pieces of armor all providing for two of your abilities or modifiers. You would have most every buff available to that subclass. That's insane. And idk much about the exotic weapons but if they're anything like the armor then they'd be beyond OP as well. The legendaries are plenty powerful and you can equip as many of those as you want. Bungie doesn't make games that allow you to become a god. Halo's in-game modifiers were called skulls, you had to find them in the game, and when your turned them on most of them make the game a hell of a lot harder. You're just going to have to decide which exotic is right for what situation, and act accordingly.