Why does the number of publications for computer science and economics PhD students differ by so much?
I don't believe what you say is univesally true of all CS areas; it is certainly not true of mine. But more to your point, based on my conversations with colleagues in economics, one factor (possibly of several) is definitely the submission-to-publication delay: A conference turnaround is about three months, but a journal can be much longer, especially if major revisions are involved.

Here's some insight on the subject (published a while ago).