Regarding the Northern/Emergency Runway/08L/26R or whatever you wish to call it.
Because its only designed to be used when the main runway is temporarily out of use being inspected or resurfaced or repaired or otherwise maintained (eg new landing light system installation) but is far too close to the main runway to ever be safely operated when an aircraft (especially a 747) is standing on that other main runway.
I don't know the reason for not using it in this instance but it can be used with an aircraft on the main runway and has been in the past.
I recall using the Northern(08L) several years ago whilst taxiing departures up to 737 size down Juliet and anything bigger, including Heavies, down the main after crossing the Northern at Romeo and using Delta to access the main.
Was a very interesting afternoon!