Why? Bananas are naturally slightly radioactive. While it’s unlikely your body could retain all that radioactive potassium in those bananas, if it did you’d absorb enough radiation to cause an increased cancer risk:
P.S. One BED (Banana Equivalent Dose) is 0.1 microsieverts of radiation. XKCD claims that 100 millisieverts of radiation is the lowest dose linked to increased cancer risk. Take that with a grain of salt … but do the math.