I think Smart makes it on reputation. I am not saying he isn't a great defender, or undeserving of a placement on one of the teams, but I don't think he has been as dominant defensively as past years and I am not sure he has had a bigger defensive impact this year than Tatum or Brown.
Of course, Marcus got snubbed a bunch of years too so if he gets the 1st team nod, I am good with that.
Tatum has been spectacular this year, especially on help, weak side defense and following defenders and blocking shots from behind. He is an amazing team defender and his one on one defense is very good and getting better. Unfortunately, unlike Smart and Brown, Tatum can get bullied by bigger stronger players down low, but he is 21, still putting on muscle, and really, is a natural SF and not a PF.
Brown has done a great job being maybe as versatile defensively in who he can defend at a high level as Smart. His one on one defense on ball is excellent. He often starts games guarding the toughest offensive player playing 2-4 or the largest most physical player. As a guard, Brown might guard PFs more than any other guard in the league and though Hayward and Tatum get listed at PF, it's mostly Brown who starts off guarding the PF position.
I think they have all been amazing, in their own ways.