The main reason is because to secure your money, we need an exit to a safe haven. In our case that would be dollars or euro's. So only exchanges that support fiat pairings are considered.
Another requirement for us is that the exchange does NOT support margin trading. Margin traded exchanges like Bitmex carry more risks as you are trading against the house. They can short squeeze or liquidate your positions basically as they wish. Besides that, we feel it's less trustworthy even though you might not use margins.
The reason why we don't use Binance with USDT pairing is that we don't trust USDT. Our safe haven, should really be a safe haven. If crypto's crash to 0, we will not be save with USDT either. Besides that, we also don't trust the team behind USDT as they have never proven with an official third party audit that every token is matched with 1 dollar.
As CoinBase is the most reputable exchange and personally have experience with this exchange with large amounts of assets, and no withdrawing issues, the decision was quickly made. I'm aware that not everyone shares the same experience as we though so we are always open for suggestions for other exchanges.
Our requirements are: