I can't point to anything specific as to why I think this, but my understanding was that the issue of O&D was a significant - but not the only - factor, when deciding to send the service off to T3. And certainly at one point, when flights like LUX, LYS and LAS were T3, it seemed to ring true. However the other factor was the timetable, T3 doesn't have the staff to manage too many services at the same time. Hence I suspect it's a combination of factors, with some trade offs involved.