As I've said before,
"So just up the rake. Sure there's a back and forth to find a profitable equilibrium, but you'd think such a stat-heavy company could work Excel enough to find it PDQ." Advertising costs, even exorbitant ad costs, are just a business expense, this kind of gambling enterprise has tremendous flexibility to adjust their margin. Even more so before all the regulatory action against them.
So, part of it is because they were running these headline contests with guaranteed prize pools. They were doing this to try to draw more users. What that guaranteed prize pool meant was that they could advertise "Play for a chance at $5,000,000!", but usually they can only guarantee that once they have enough players to grow the prize pool to that size. Instead, they were basically fixing the prize pool ahead of time and hoping it was enough of a draw to bring in at least $5m worth of entries. If they fell short, the company covered the difference.
They also ran a bunch of promos where they did stuff like "deposit $25 and we'll credit your account with an additional $50", which was meant to be incentive to keep them playing in hopes that they'd deposit more real money once they spent through the first stack.
This combo could particularly hurt them, where people using the deposit promos would play in the guaranteed pool contests.
They have to be constantly drawing more and more users, and the methods they used to try to get to the mainstream market were both costly in terms of money and costly in terms of attention to a business model that was likely illegal.