He did so to provide cover for his baseless accusations against GWB in the OpEd he published in the NYT's.
If you do a Google search on Valerie Plame you will see that Joseph Wilson gave two interviews in which his wife was mentioned, niether of these stories claim that the info came from any reliable source in the White House. One was by Robert Novak and the other was NEWSWEEK I believe.
Do you really have a link to an interview with Wilson in which he originates the story that his wife worked for the CIA?
That's not the way David Corn describes the origin of this story. He says that "two senior administration officials" were Novak's sources for his statement about Plame. Here's a link.