My impression was two things: (1) he had a mission; (2) he had strict and unusual ethical rules for himself, which precluded most other work.

RMS wasn't the only one with ethics that was a barrier to getting rich, however. One odd thing was that, at the start of the dotcom boom, I had the impression (perhaps mistaken) that the programmers with the most experience with Internet and such were also disproportionately more inclined to see themselves as bringing some altruistic vision to everyone, and nurturing it. Things like privacy violation would not fly, I don't think. The privacy violations were snuck in, initially, and the original visionaries either weren't in a position to stop it, somehow didn't realize it was happening, or simply took the money.