How did TCS become conference-oriented rather than journal-oriented?
While I don't know the answer to this question, it seems crucial that the phenomenon isn't limited at all to theoretical computer science.  I believe SIGGRAPH plays the same sort of role for graphics, NIPS for machine learning, ISCA for architecture, etc. that STOC and FOCS play for theory.  Yet it's true that the emphasis on publication in conference proceedings is a striking feature of computer science as a whole, one that isn't shared by any other academic field that I know about.  (But maybe there are other such fields?)