Reason #1 Against Trading For Johnson: Doesn't Fit Their Defensive Style
Domantas Sabonis is an incredible offensive player. However, he does have a very major limitation as a defender: rim protection (15th percentile in block rate among centers last season, per Cleaning the Glass).
Now, you can still build a great defense around a center who is not a strong rim protector. But to do so, you need to make sure that you get as much rim protection from your other positions as possible.
The Kings know this, and you can tell they are trying to do this by accumulating as many positive positional rim protectors (Devin Carter, Isaiah Crawford, Keon Ellis, Malik Monk, and Keegan Murray) as possible.
Unfortunately, Johnson is not someone who could aid in this crusade. Despite being 6'8.5 with a 6'10 wingspan, Johnson is a poor positional rim protector, only placing in the 24th percentile among forwards in block rate last year.
This doesn't mean Johnson is a bad defender (he was in the 54th percentile in Defensive Estimated Plus-Minus). He's just not the kind of defender who can really help this Kings team.