I have flown to both Sandown and Bembridge frequently and must say that I much prefer Bembridge for its facilities, very welcoming radio operators, restaurant, scenary and runway quality and its the same price to land as Sandown.
Bembridge in my opinion offers much a much better experience for the average GA pilot and it seems that Sandown has nothing better to offer.
Maybe this is a reason?
Obviously everybody is entitled to their own opinions.