I wondered that about bananas too. Maybe because it is a tropical fruit and is usually best to eat in the summer months. The body can take a longer time digesting things in the winter months since people are not in the Sun as much. Sun helps to fuel the digestive fire. So it may be a concern of fermentation in the gut that could lead to candida (which cause us to store more fat due to the toxins and estrogens they produce).
I do like bananas too. I will still eat them when I feel the time is appropriate. Prunes and oranges are also a great source of potassium. Most vegetables have a lot of potassium too. I juice vegetables and fruits which helps me keep my potassium level stable. I was deficient in potassium a few years ago, so I always think about potassium too.