There's a lot that goes into space worthiness rating even beyond the technical spec requirements. There's probably a bid request available for when they were first considered too. No idea where to start hunting that down though.
There was a really good article[0] (plus a video talk I can't find) on a team that modified an Android phone to go to space to interface with the SPHERES mini satellite experiments. A short list of the things they had to do includes:
- No Lithium Ion battery, it takes 2+ years to get a LIon battery certified for the ISS
- Had to put a screen protector on, broken glass screen becomes an inhalation hazard in zero G, BUT many materials are considered flammable in the high oxygen environment on station.
- Had to lobotomize the Wifi and cellular chips to ensure they'd never turn on. Just removing the software that would control and allow them to turn on wasn't enough.
In addition to everything in the article they have to worry about off gassing from all the various materials that make up anything sent to space.
Requirements include thermal management (ie. cooling working in microgravity environment, without convection), power supply compatibility and EMC.
I assume that the thermal management part is significant reason for use of thinkpads. On another brands one can see that cooling is significantly dependent on orientation (usually with the best orientation for cooling being something other than "standing flat"), while thinkpads are able to get incredibly hot in any orientation, but I've never seen one overheat.
Another thing is the whole reliability and serviceability thing.