Its very simple.
If you have enough visitors to generate the amount of business the casino expects to make for the investment, you’re in and will get media buys.
If you sell space and the casino neither sees the huge numbers of impressions required for straight media buys, nor sees the clicks it would like to see, you get dropped and they will avoid you in future and word gets around about you not performing.
I do both, media buys and affiliate deals. With many programs I am happy with the affiliate deal. You are ignoring that, if you are in it for the long run, you will build a player base that is loyal to the casino. The longer you do this, the larger the income.
You are also ignoring whales. A few large players will outdo your flat rate many times over.
Flat rate is good for security (if you can live up to expectations).
I get several media buy offers every month and accept maybe 4 or 5 a year.