There is just too much risk involved in flying a circle in low weather. We've all heard about good landings being preceded by stable approaches. Well, a circle is anything but a stable approach. It is something we do very infrequently, therefore it is very hard to keep it stable. Not to mention that you will most likely be flying manually while trying to keep visual with the airport, often taking configuration changes at non-standard times. Today's airline procedures and cockpits are just not designed to be "circle friendly".
Harry Truman said is best...'can't stand the heat, stay out of the kitchen.'
