Earlier quoted context omitted.
Reddit should try honestly. "We are losing lots of money, we need to start making money, reddit gold isn't bringing in enough revenue to pay the bills. 3rd party apps don't show ads, which costs us a lot of money every month. Keeping the 3rd party APIs up and running also costs us money. Because Reddit needs to stop losing money, we are closing down 3rd party apps." I don't know what why it is so hard to say that...
Reddit had an easy way out for the issue of 3rd party apps not showing ads, they already have a paid subscription which removes the ads on the official clients, so they could have made the API exclusive to users with a subscription. People would have been upset, but not this upset.
Apollo is shutting down because the founder thinks they'll incur about $2.50 per month of costs per user, and apparently doesn't believe enough people will be willing to pay $5 monthly to keep Apollo running.
So, this Reddit Premium (billed at $5.99 monthly) either has few-to-no paid users, or Apollo's founder isn't even trying to sustain his business.