Why is an accident referred to by flight number instead of registration number of the airframe?
Most accidents are referred to by flight number


It depends who it is that is referring to the accident, and on who they are addressing.

News media and the general public

Newpapers and other media use flight numbers because that is the number known to friends and relatives of the individuals concerned and because most members of the public who have flown as passengers are very familiar with  the concept of flight numbers but not really familiar with aircraft registration numbers.

Investigators

Investigating authorities have their own numbering schemes for accidents they investigate.

Aircraft registration numbers change when an aircraft is sold to a purchaser based in another country.

The only number that stays with an aircraft for its entire life is the manufacturer's serial number.

All these numbers can appear in official accident investigations reports. The registration number is usually prominent.


AAIB


NTSB


BEA