Speculation: Without doing a deep dive into their wallets, I would assume there was an exodus when they prohibited US signups last June and officially kicked out US customers in September. I wouldn't be surprised if Americans previously comprised 30-40% of their holdings.
That's certainly possible, but the odd part is that Binance in the last 12ish months has been moving all its cold wallet holdings back and forth like a dozen of times, which not one single other exchange does.
Another odd part is that despite US traders no longer being able to use that platform, the volume on the BTC/USDT pair hasn't gone down much at all. One would expect the volumes to see a very firm decline if US traders compromised 30-40% of their holdings.
Bitfinex's past exodus was not only evident from their cold wallet holdings that were shrinking month after month, but also from their much lower volumes, which makes sense if fewer traders are left on the platform. Binance either artificially inflates its volumes, or some other shady stuff is going on there.