It took long enough
Sony has always had a nebulous contract with Bungie (and by extension, Activision) regarding Destiny 1. Several items, maps, and strikes (dungeons) were timed exclusives for the PS3/PS4 versions of the game, but how long that period lasted was completely up to a closed doors meeting. In some cases, content made its way onto Xbox platforms in roughly a year, and in others, Bungie simply delayed it on Xbox until they felt like bringing it over.
Now, Xbox players can access the Taken King content Echo Chamber strike, sublime armor, a quest from Petra Venj, the PVP map Sector 618, the Jade Rabbit exotic scout, and the Zen Meteor exotic sniper — a whole two years later. By proxy a Tyra Karn quest, Iron Camelot armor, and the PVP map Icarus have also been dumped into Destiny 1 on Xbox from Rise of Iron, which didn’t take nearly as long to arrive.
To drive the point home of not having a clear deadline, Destiny 2‘s exclusive content is set to only stay on PS4 until “at least fall 2018.” Again, note the “at least,” as they could easily just delay it again.