Complain to my heinous editor, Mr. Wesley Adams at Farrar, Straus and Giroux. He made the choices--I merely held my breath and approved whatever he wanted. I was happy some company was willing to take on such a project. See, for publishers, bringing out some well-liked books collected in an inexpensive paperback edition is as daring as space-walks. If enough copies sell, yes, there will be another collection. You can help this happen by giving copies to persons known to you, who can read, as holiday gifts. (Cheap ones, too).
I think Adams's idea was to provide a sampling. Probably, he did not dream there might be another collection, and expected to get fired after talking his company into risking more than $4,000 on such a radical project.
My own wish was to make it a really overstuffed 2-volume set with about 20 out-of-print books--and I offered to let FSG do so for the same paltry sum they ultimately advanced me--but it was just too scary for them. I am delighted 5 Novels exists, and hope it does well enough for them to bring out another.