According to Nintendo in a statement to Polygon, “The delay is due to the preparations for the Pokémon Go application update in order to optimize Pokémon Go Plus. In order for us to offer new experiences to play Pokémon Go with Pokémon Go Plus with a quality level high enough to satisfy as many people as possible, we decided that it is necessary to take enough time for the completion. We ask for your understanding.”
Basically it sounds like Nintendo was waiting for the Pokemon GO app to be updated to ensure it played nicely with the accessory. Prior to this, some had speculated that maybe the delay had to do with production issues, but we guess this isn’t the case. Either way the accessory is still on track for a September release so you’ll have a month plus or so of waiting.