I agree, Castle is pretty easily the best of the bunch, with the benefit of hindsight. But the thinking at the time is not a mystery. It's pretty obvious what the thinking was with each of those guys. And it wasn't a slight on Castle, who went #4.
Risaccher/Sarr - Bigs who can stretch the floor remain a much sought after commodity in the NBA. Add in the recent reputation of French prospects in NBA circles, and you can easily see the thinking here.
Sheppard - Shot over 50% from 3 on decent volume (4.4 attempts per game). In the modern NBA, that's going to get GMs salivating. Nobody wants to be the guy that missed on the next Steph Curry (who was not as accurate as Shepperd in college, though his volume was significantly higher).
Castle just fits a prototype that's more readily available in the league. We can see now that he's a particularly good example of that profile, but that wasn't apparent at the time of the draft. I don't think that was a particularly great draft and I don't think there was anyone in it that was a slam dunk. And I remember that was the popular opinion at the time as well. So if you're going to take a flyer on someone, it make sense you'd take a flyer on someone in a more sought after, less readily available prototype.
And these guys haven't exactly been bad. I'd argue they've been pretty good, or at least shown promise. FWIW, Sarr posted a better PER than Castle this season and Shepperd edged Castle in win shares. I will say I think Risaccher looks less defensible than the other two so far and I have a feeling that, particularly as the #1 pick, history will likely look on him as a bust.