You can blame Luka Doncic, the Mavericks third overall pick this year. He has played so well that the Mavericks believe he is their future. Nobody will argue with that statement. It is true. He has shown great potential and he is only a rookie.
Making Doncic the franchise players means getting rid of Smith Jr. When both of them are on the court together, the Mavericks suffer. The team has realized that and they have chosen to build around Doncic and trade Smith Jr. away.
Trading Smith Jr. right now is too early considering he is only in his second year. But that also means that teams will give up a lot more to get him. He has a favorable contract and he has shown potential to be great.
It isn't fair to Smith Jr. but that is the NBA for you. One day you are the centerpiece of the team and the next day you are being traded away. It is the best decision for both teams. Smith Jr. can join a team that can make better use of him. The Mavericks believe that they aren't that team anymore and are doing him a favor by trading him away.