Computer science conferences are much more selective and the quality of one's work is measured by which conference the work appears. Journals play a far lesser role and many important papers never appear in a journal at all. Why is computer science different?
The answer is technological, namely airplanes. Before air travel conferences were much more difficult to attend and drew from a much more regional audience. Those who made the great effort and time to attend a conference were allowed to present. But presenting your paper at such a conference would not reach the majority of your colleagues. Journals were the most efficient way to broadly publicize your research and took on the more important role and have kept that role for historical reasons.
Computer science started as a field during the jet age. Many more people from a wider geographical base could attend a conference. One could now widely disseminate their research through conferences well before a paper appeared in a journal. Journals still played an important role for refereeing, editing and archiving but never held the importance in computer science as conferences do.
Since then we've seen another technological revolution and the internet easily trumps conferences for quickly distributing your results. Perhaps some new scientific field starting today would have a different internet-based system for judging research. But conferences will remain the primary focus for computer science as journals do for the older scientific disciplines.