It is unclear why Square Enix has opted to make such a decision, but for gamers who don’t necessarily have a good internet connection or plan on playing the game offline, then this is kind of a bummer. Alternatively the company might not necessarily view the epilogue as being essential to the game, at least based on their wording which says, “The epilogue and secret movie are video content that will enhance player’s enjoyment of the world of Kingdom Hearts.”
This means that if you’re perfectly fine with how the game ends as it is, then the epilogue will only serve to enhance that, but at the same time it’s not necessary if you already enjoy it. Also note that the epilogue is separate from the main game because Square Enix claims that the ending of the main game can be viewed without an internet connection, so this is just bonus content.
*The Memory Archive is a digest of the #KingdomHearts story in the series up to this point.
*The epilogue and secret movie are video content that will enhance player's enjoyment of the world of #KingdomHearts.
— KINGDOM HEARTS (@KINGDOMHEARTS) January 11, 2019