Seapy, makes you wonder why in 300 years nobody has opened up the window again?
Good point Mike, I have seen it many times where a window has been blocked up but never re opened.
All I can think of is that the owners have been reluctant to mess with it, getting matching glazing, particularly with mullioned windows can be both difficult and expensive. In some cases walls are more useful than windows when it comes to furniture placement. In other cases it may be that the blocked up window has had an internal wall abutting to it to reduce one larger room into two smaller rooms so it would be undesirable to reopen it.
There is also the thought that in some cases the feature adds interest to the property. In this case if it were mine I think I would reopen it because it's quite small and low, the room must be very dark and I like nice bright rooms as a rule.
