Because the Supp Proc in FCOM tells you to only shut down using the ENG FIRE pb if the LP fuel valve is not closed. By using the ENG FIRE you have now disconnected a whole bunch of other systems including FADEC. Now engineering will need to attend the aircraft,which if you are not at an engineering port becomes an issue that could have been avoided if the engine was allowed to shut down normally albeit a little bit longer than normal.