"I thought we had a right to die...."
Obviously, one reason is: to get bigger numbers. But I think the people that lump gun deaths together believe (or want others to believe) that guns are really dangerous. When it comes to suicide, there are 2 ways to think about the deadly effectiveness of guns: 1. For those who really want to kill themselves, guns are a sensible choice, or 2. The scary deadliness of a gun tempts weak/impulsive persons to go ahead and do something that wouldn't happen otherwise.
You can easily see that those 2 ways to think represent the mindsets that lead to libertarian or authoritarian answers to all sorts of questions. #1 would allow the individual to make his own decisions and to take care of himself, and #2 thinks the individual — call her Julia — needs to be helped and protected (even from herself).
Sorry to go all gender-y, but I'm interested in talking about suicide and attitudes about guns in the context of gender difference, because 4x as many men as women commit suicide and 56% of male suicides use firearms compared to only 30% of female suicides. Those statistics are skewed by the fact that guns are an effective method. It might be that the gender disproportion is because men choose the method that leaves fewer survivors of attempts at suicide. I note that 40% of female suicides use "poisoning" (presumably, that includes drug overdosing). What's the proportion of females attempting suicide by poisoning to females succeeding in killing themselves with poison?
If you have a fantasy of rescuing those who are in the process of
committing suicide, you might think taking guns away will give you a better shot.
ADDED: It occurred to me, after the Sandy Hook murders, that blaming guns is a secular substitute for blaming the devil. People find it too challenging to figure out why a human being would do this terrible thing and they latch on to the idea that the gun made it happen. Suicide presents a similar challenge, and one way to fathom it is to say: It was the gun. Isn't it like saying the devil made him do it? The gun/the devil is a great go-to answer, freeing you from wracking your brain about the workings of the human mind.