Because Excelsior has been overpopulated and experiencing database issues for the past two years. The Homecoming team has done a great job of trying to mitigate the problem (by moving some of the database directly to the server's RAM for example), but haven't solved it. See the shard's occasional database lockups causing queues during peak activity times like Rikti mothership and Hamidon raids even if the shard has under 1,000 players on, and regular server lag. That became greatly exasperated over the past couple of days. For the good of both Excelsior and Homecoming as a whole, some players need to move off from Excelsior to other shards. Better to offer a carrot of double XP and Rikti invasions than a stick.