This service has only operated regularly on weekdays for about 15 years - it used to only go at Olympia exhibition times, which I think continues.
I'm sorry, but it is invariably almost empty, it is a great waste of resources for what it must cost. It's not that frequent either, and as buses from in front of Olympia to Kensington High St station and beyond are continuous, it is an obvious one to reduce expenditure.