One answer I don't hear used often concerns account hacking and credit card fraud. Those engaged in the practices want the ability to dump their ill-gotten gains quickly. If a game doesn't have a huge market for illicit game currency, then why bother hacking the game accounts? If the market is small, then those committing credit card fraud won't have the ability to launder large amounts of real life cash. I know, some hackers will hack just for the challenge or the lulz. But taking away some of the incentives does help protect against the ill-intentioned.
From monitoring various ISK selling websites, I have a fair idea of how much someone can make selling ISK. The fact that this seller was essentially working a part-time job of probably 15-25 hours a week, making $300 USD might not appeal to a lot of people in the U.S. But in some countries, $300/week is a lot of money.
The final answer, to a question about why he stopped, is a bit illuminating. So much so that I'll repeat it again:
"I lost interest in playing and coupled with the drop in price for ISK, it really stopped being worth it."
CCP's goal, as I've stated several times over the years, isn't to catch and ban every person engaged in illicit RMT and its associated activities. The goal is to convince people that selling EVE Online currency is just not worth the effort. For one former player who posted on r/eve yesterday, that was certainly the case.