Because thats what most "gun related deaths" are-- suicides.
But it pads the statistics to make people who aren't paying close attention more fearful to talk about "gun related deaths" than "murders with guns", and it also helps avoid the fact that the murder rate in countries where guns have been banned is much higher-- even if they have to use knives to do it. (how often have you seen the murder rate in Britain brought up vs. the number of "gun related deaths" brought up there in this debate?)