It (word limit) is established by the bookstores, who limit the number of inches a binding can occupy on the shelf. Any author whose books do not sell through finds his name not on the list of books the stores are willing to buy from the publisher. Nowadays, now that bookstores are large conglomerates, the calculation is done by computer with no human judgment involved, so if a midlist author with a fine record of book sales suffers one book that was bought in too large a number or which made too small a sale, suddenly his books are not bought in as large of numbers by the distributors, and therefore his sales drop again, and therefore his books are bought in even fewer numbers, and therefore his career is over unless he changes his name and writes under a pan name. This has happened to a friend of mine, John Hemry/Jack Campbell who writes the LOST FLEET series.
What happened to me with THE HERMETIC MILLENNIA is that a staffer made a mistake and over-ordered the production — whether it was too many units or the manuscript was too long was not clear — and so I cannot make my numbers unless I sell out every single copy of the book. That is why I am pathetically begging my readers to go out and buy them all, because I have been informed the publisher will not buy the rest of the series. If the book sells through, I have some hope that they may reverse that decision.
Scott Adams wrote of "Powerpoint poisoning" and this smells like Excel bookstore management. Just plug in the numbers and you're guaranteed success.
I've read several of his books and enjoyed them, btw.
UPDATE: Changed the title. Professional as in makes money from it. I liked the ambiguity of real writer/real problem, but it doesn't work well.