Why conferences are the main venue for CS research?
There's a reason CS folks cite when we obsess about conferences. The claim is that the field moves so quickly that conferences are more effective than journal for fast turn-around, and so better reflect the speed of developments. 

I think this statement is partly true (conferences do have faster turnaround than CS journals) but misses the point entirely (there's no reason journals CAN'T have faster turn around time). 

The real reason is the usual one. We got used to having conferences be the primary source of dissemination, and have no pressing reason to change. Having said that, the arxiv is more and more becoming the first choice of reading material and "hot off the presses" material, so I suspect that your question will become more and more moot as time goes on.