I noticed a sort of almost prurient interest in salacious details about ISIS among liberals around the time the podcast came out, so I think that was part of it, they just knew something along these lines would be popular and sell. (Conservatives may have had much the same interest, I don't know, but if they did they weren't going to the NYT to get their fix.)

The "ISIS as unconscionable bogeyman" story also helped sell the U.S. intervention in Syria, which was an ongoing objective on the NYT's part.