Near the end

I have been photographing this old building for the last 15 years. It's known locally as The Old Barn, and it's generally held to be the oldest building in the area after the local church. It's probably 13th Century and may have once been a chapel. 

The last time I saw this it still had 3 walls, and as recently as the 1970s it was a recognisable building: http://www.geograph.org.uk/more.php?id=3458778 http://www.geograph.org.uk/more.php?id=3458778 but just look at how it is today.This is a scheduled ancient monument. How has this happened to it?