The reason why you cannot participate in the FUTU Cash Sweep Program (「Program」) when you are marked as a pattern day trader (「PDT」) is that the cash in the Program cannot count towards the $25,000 of minimum equity that a PDT is required to maintain. Cash in the Program therefore, will not count towards the day-trading buying power. This measure follows standard industry practice because permitting a PDT customer to participate in the Program could result in continuous day trade calls for the customer. Please visit FINRA's website for more details about day trading.
Please note that if you are marked as a PDT while enrolled in the cash sweep program, you will be unenrolled from the program and the cash and already-accrued interest will be paid to your brokerage account. You will not earn interest on the uninvested cash that is not in the Program.