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
In a sense, this is the logical evolution of the conference. If you want a place to disseminate results quickly to a wide audience, what better way to do it than via the e-print server, especially given how active it has become. Physicists still publish in journals as the primary source, but e-prints have helped replace some of the functionality of the conference.
Conferences will really never go away, since there is no replacement for direct one-on-one contact in my opinion, but e-prints are definitely a new Internet-only model...