Why doesn't Viking publish the rest of them?

That the "Penguin Proust" translations aren't all available in the United States is thanks to the manic American copyright law, as mucked up by Sonny Bono at the best of the Walt Disney Studios. For fear that Disney's odious mouse would fall into the public domain, the term of copyright was extended in a way that would keep Mickey the property of the company for the foreseeable future. Naturally, this had consequences undreamed-of by the singer turned California congressman.

Bono's legislation effectively froze the date at which works go into the public domain at 1923, instead of marching forward twelve months with each passing year. In Search of Lost Time was caught in this web, because only the first four volumes were published before 1923. The Prisoner, The Fugitive, and Finding Time Again are still protected under U.S. copyright law, so Viking can't publish them, even though the British paperbacks are freely imported into this country.

Of course, Viking could have contacted all of Proust's likely heirs, but its legal department probably advised against taking this step on the grounds that someone might be missed, only to turn up later accompanied by a lawyer. Or perhaps the cost was simply thought prohibitive--what if the final heir held out for millions of dollars? In any event, Viking decided to wait until 2019, when the U.S. copyright on The Prisoner will finally expire. It remains to be seen whether the company will still be interested the project at that time.

The Copyright Term Extension Act of 1998 is variously known as the Sony Bono Act and the Mickey Mouse Protection Act. Under whatever name, it was certainly a Mickey Mouse piece of legislation.