Why was the publisher embarrassed by a rare bestseller?

I don't think so. I am assuming the books by the best writers, close friends, their writing liked by him, didn't sell particularly well. When he did ok a book that became one of the firm's relatively few best sellers, he was embarrassed because he didn't admire the writing.

Personally, I find Sontag hard to read, but she was a well known and often admired intellectual. I've read some of Brodsky and was engaged with his work, would like to read more.

Sammy Davis was a film and tv star, not known for writing (I don't know if he had a ghost writer involved or not), and Turow is a readable guy who wrote legal procedurals.

These are different kind of writers and Straus knew it.