Why conferences are the main venue for CS research? Based on @JeffE's suggestion here, I see many CS researchers care mostly about conferences deadlines. Even when they want to read papers, usually they check the recent proceedings of different conferences.  

Having a paper in IJCAI or AAAI for example, worth more than publishing in many ISI indexed journals with good reputation. I have no hard evidence for this but being in touch with CS research, I see little discussion about journal publications. why is that? is it good for the spirit of research in the CS field?
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.