Maybe there's so much work involved with creating it that there's little energy left over for arranging display opportunities.
What Andrew said seems about right to me.
I have been involved with a number of professional painters throughout my life, and I must say I am jealous of the time they have available to do things such as visit each other, teach classes in a nearby college, go on lecture tours, or just sit around their studios and read books; whereas sculpture just doesn't seem to allow for that. It's so damned labor intensive.
The big time sculptors—Louise Nevelson, Jeff Koons and Rodin come to mind—have, or had, teams of workers doing much of their work. Koons is more of a promoter and snake oil salesman than he is an artist, but, "Hey!" no artist I can think of is a more perfect match for his time.