The reason they do it through Xur is because, like most of their other decisions, they want to artificially extend gameplay. Xur will only come once a week and people will get all excited each week hoping Xur will offer their exotic for a trade. It's a shitty RNG based system that keeps people playing like everything else. People will keep playing in the hopes that their old exotic they want to use in the new hard mode raid will be available. They could make the gunsmith also only offer the exotic trades on Friday and Saturday but "out of sight out of mind." If the gunsmith was sitting there Sunday-Thursday and wouldn't trade in your exotics people would complain that that's stupid. He's right there, why can't he do it. Xur is literally not around to do it so it plays into peoples psychology such that they won't get as upset about waiting a week to do the trade. It's all deliberate. It's mostly bullshit.