In general, if a startup fails, that's part of the deal. The entrepreneur loses thousands of hours of work. He doesn't owe anything to his investors.

What's different about Evan's story that compelled him to repay them?

There's nothing honorable about buying high and selling low. It's bad business and if this were the creed of our "model entrepreneurs" the world would be a worse place indeed.

since when do 'honorable' and 'stupid' mean the same thing?

From what I gather it seems like he wanted out of the project. At some point it seems he decided "I'm not really a podcast guy, I'm a Twitter guy" [not a quote]. He did what I think was a pretty honorable thing and took the hit himself. He could have easily shared the loss with his investors. I would of course agree it was risk they signed up for, but he chose differently.