Business Decisions.
"The venerable line of PC notebooks rolled onto the scene in 1992. While the concept was spot on, there was turmoil at IBM as to what to call it. IBM's pen-computing group wanted to keep it simple; they liked ThinkPad. But IBM's corporate naming committee didn't—it didn't have a number, and every IBM product had to have a number"
Cio.com
It's hard to expect any kind of innovation from a corporate culture this conservative. Luckily they managed to decide against bureaucracy.