he didn't sell the company. he sued the company and they settled out of court. he made his money via legal action, not business or programming acumen. the sale was for pennies and was only done so that the VCs wouldn't look completely retarded for investing in the first place.
he didn't really have any choice but to retire. no investor will give him money and no programmer with any sense would work for him.
Greenspun had other companies before Ars Digita. He took ICAD/Concentra public and also co-founded ConSolve and Isosonics.
It may be true that no programmer with any sense would work for him - I don't know him personally. But given how cheaply you can start a company nowadays, he certainly doesn't need investors. And as I understand it, he's not entirely retired: doesn't he teach a course at MIT?
I don't know - "may be true" is just conceding the point to the grandparent, because I have no personal knowledge of Greenspun, and from his username, it appears that the grandparent worked at Ars Digita. I've heard that Greenspun's management-style was pretty abrasive and he could very egotistical, but this is all hearsay.
he didn't really have any choice but to retire. no investor will give him money and no programmer with any sense would work for him.