Probably because they don't understand and might they didn't follow the step-by-step setup of the kickback.  I didn't an expert on this but I think Binance will not ruin their reputation on this referral program they had.

Just try to read this article posted by the Binance website, the Referral program FAQ, this might help you.

Comparing both exchanges I prefer to choose Binance but for me, they are both have good reputation, the only advantage thing is that Binance is a more popular one than Okex.