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.
The lack of a trackpad has to deal with not compromising space for the keyboard.
Ah yea, I did have a T series. Thanks for the info.
Probably an over-summary but basically correct: the X series optimised portability and only had a trackpoint, while the T series optimised fully-featuredness and had both trackpoint and trackpad.
I still fire up my T61 from time to time -- it's the only computer in the house that still has a DVD drive.