How did TCS become conference-oriented rather than journal-oriented? Disclaimer: I can only vouch for my research fields, namely formal methods, semantics and programming language theory. The situation is possibly different in other parts of the discipline.

It seems that TCS has become rather conference-oriented. Researchers aim at publishing in the next conference. Sometimes a journal version appears. Sometimes it doesn't. 

In other disciplines (biology, mathematics, and most others I guess) this is unheard of. The effort put into writing the conference papers is a lot lesser, but in turn, the conference papers count a lot less. The "real deal" is the journal publication.

Arguing whether this situation is good or bad could result in a flame war, and doesn't have a precise answer. Instead, let's try a more factual question:

How did we become so conference-oriented? How did conference papers gain so much weight?
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?)