It's that he physically can't shoot 3's at a high clip (with his FT %/form, shouldn't be surprising). Contrary to SpursTalk belief, teams don't necessarily want or need every single player to be shooters first, and everything else second; unless you can really shoot, it's worse for the team if your center spends possessions on end inefficiently chucking at the basket, creating fastbreak points for the other team and wasting possessions, than it is to simply have him do what he does best, and leave the shooting to your actual shooters.
It's the same reason Gobert doesn't camp out the 3pt line, even though he's routinely seen draining 3's in practice. Practice =/= game.