Yes, I appreciate that ATC will need to reroute and advise the new destination airfield, but why do they need a reason for a (non-emergency) diversion?
When I have had to divert for non-emergencies, the real reasons have been banal. Once the boss's secretary had omitted to warn me of a last-minute change of destination and I only found out when the boss tapped me on the shoulder and asked why Paris (his new destination) was disappearing behind the left wing 45000 feet below. On another, striking Spanish ATC held us down at FL250 right through Spain meaning we were burning too much fuel to complete our flight which was planned at FL410. On another (in a toiletless aircraft) the pax was caught short. I always hesitate to give the reasons for the diversion in case ATC refuse.