At the time X61s was notably smaller and more expensive than its competitors and other models in the Thinkpad lineup. The "normal-sized" model T61 has both touchpad and Trackpoint, so leaving touchpad out may have something to do with the case dimensions. Maybe there were no space left for it or it would have been awkwardly positioned when using keyboard or Trackpoint, and on a more "special" model leaving it out has been a valid option. After all, T61 was available for customers absolutely requiring the touchpad.