"It's just ridiculous, there's no reason for this game to have done well. It's nice for everyone involved," McMillen said in an interview with IndieGames.
"When I started development on Isaac, I wasn't even sure if I should charge for it, because I didn't think people would want it, in all honestly. I had to shop it around to a bunch of different developers, and say like 'Do you think I could sell this?', because I thought it was way too weird, I thought the content was too disturbing and creepy. I thought it would just rub too many people the wrong way and I thought the design was just too hardcore for any kind of mass amount of people to enjoy."
Due to the success of Isaac, McMillen and crew are currently working on The Wrath of the Lamb expansion, which will be $3 and launch "when it's done."