Their hosting costs I'm sure are astronomical; my guess is that honestly that's most of it.
They're also, if my very limited experience with them is any guide, phenomenally incompetent with their advertising in a way that I'm sure kneecaps what should be a goldmine of ad revenue. You know those brain damaged ads like "Megathread: Why you should move all your money to Schwab" or otherwise trying to imitate Reddit terminology in the least convincing way possible? That's because Reddit tells their advertisers to do that. For real, it's worth looking over Reddit's ad materials sometime, because they are pants-on-head mentally disabled in a way that's honestly a little hard to believe if you haven't checked them out for yourself.