Notice how a lotta these tornadoes are from the plains/prairies? Well, within those regions we see zonal, shallower, or more mesoscale based setups; tornadoes/storms from these types of background environments are slower. As we see hodographic profiles favoring deviant motion, boundary riders (think Jarrell or Matador, same for Sterling City!), and weaker shear environments which favor CAPE based setups; this is how your "grinder" tornado is born, slow movement and extreme instability. Almost every single slow moving vio-tor can be traced back to these very principals; Harper, Jarrell, Clyde, Matador, Cisco, Duke, Loyal Valley, Bakersfield Valley, heck even the Lubbock F5; they all were set in environments which appear in these regions. You don't see these slow moving grinders in dixie, the genuine evolution of mid-latitude cyclones is why!
Good rule of thumb for history