Every exotic will be upgraded at some point. They have to not only redesign how they look, but also change how they function so they can compete with the new exotics, that takes a long time. For now, they're just upgrading a few favorites like Thunderlord and The Last Word (While leaving out weapons they feel are overused like Gjallarhorn and Thorn) and the PS exclusives so they're not useless when Xbox users finally get them. More exotic upgrades will be released over time, you can stop now.