While I think the referral code is part of it, BNB is large to. On other crypto forums/groups you always see Binance mentioned, without referrals, but with "make sure to buy some bnb to reduce fees!". Its the easiest coin to shill, as you are giving legit advice.
The other part, of course, is iota. They had it, others didn't, it 4xed .
And lastly, its obviously the only exchange you should hold neo on, as they actually payout the gas.