Probably because Tool Pallets are not set up OOTB for command usage, for the most part. Sure there are -some- command sets on the tool pallets, but not really as a main interface.
Plus, whether you use the Ribbon or Toolbars or Keyboard, these options all have one thing in common... you don't have to click around to find the command you want to use, most of the time, for routine usage. The Ribbon being the worst of the three of these still has -contextual- tabs that pop up which serve the purpose of putting the commands you will most likely need, in yo' face, right when you'd want them.
Toolbars are always there, so require no "page flipping" to get to them like pallets do.
Pallets are also extremely... and I mean -extremely- inefficient at using screen real estate. Look at the square pixellage real estate that one command would take up on a pallet and compare that to an icon.
I will leave the advantage of keyboarding out since this is likely a GUI-focused topic.