Many trade publications — as well as academic publications — don’t pay contributors.
The reason is that the people who contribute are getting something other than money in return: free advertising. For a lawyer, CPA, engineer, or other professional, the exposure may well be worth quite a bit.
However, editors have to spend quite a bit of time recruiting these experts and holding their hands through the writing and revision process. While writing is a joy to some people, for many people it’s an odious chore and a reminder of bad old school days and essay assignments.
Finally, the results are often unreadable and editors have to spend quite a bit of time getting the pieces into shape, or risk alienating readers. In effect, the publication is padding its pages with advertorials. And if a reader has a choice between a well-written piece that quotes an expert, and a barely-readable piece from that expert himself, readers will often choose the piece written by a real writer.
As a result, expert-written publications often have limited markets, or are distributed free to readers.
Moreover, experts — while they are able to write a column here or there on their areas of expertise — are rarely able to consistently produce work. They are also notoriously bad at meeting deadlines — after all, if they have other, paying, work to do, the free articles will go to the bottom of their lists.
As a result, some publications do a combo approach — use experts for occasional columns and writers for the material that just has to be in, and just has to be readable.
I occasionally write for publications that are partly expert-written and enjoy high word rates (probably the editors appreciate what they’re getting).