Because BU has a goal of replacing BTC asap, and since both will have the same blockchain history until the moment of forking, BU will be able to easily appeal to most of the Bitcoin users. Altcoins are usually distributed via mining starting early from genesis block or ICO, but BU will be different from any altcoin, in terms of distribution. So in some sense BU is an altcoin, and in some is not. Just look what happened with ETH and ETC and you'll get some idea of what can happen with Bitcoin and BU.