IANAL, but they may have structured their investment as senior debt, and depositors may have some other funny legal status. Also Alameda has to be dissolved before FTX, so technically FTX depositors are a creditor to Alameda while the Alameda investors are depositors, and typically Chapter 11 discharges debts. Also, Alameda is under Bahamian law, and so is FTX, and US courts will only get what's left.