Donald, I've often wondered about that. It seems to me that a longer tonal scale and a lower dmax are contradictory, as a longer tonal scale to me requires a higher dmax.
In any event, if you make your own emulsion you can control the tonal scale rather than take what the manufacturer thinks you should have.
You are correct about contrast, I had forgotten that, but can you devise a range over 4 grades? You can with silver. IDK personally if you have that range with Pt/Pd.
And, you can keep the emulsion for quite a long time, as well as the coated paper or film. Can you do this for Pt/Pd? I really don't know.
I'll probably think of more reasons, but cost comes to mind as well. Silver nitrate costs a LOT less than either Pt or Pd salts.