Not everyone has someone that they are going to be leaving behind. Given what some of the users in this community have posted it seems that they are the ones who have been left behind. That is why they wish to die, because they have no one left to care for.
Also: why is leaving others behind considered bad in the context of suicide?
You have a boyfriend or girlfriend, but then they break up with you. You have a family, but then they disown you. You have friends, but then they move on when you are no longer useful. These could all be considered examples of being abandoned by other people, but we do not see any of them as wrong necessarily. In fact those actions are defended. After all: "you cannot force someone to be in your life" is what is always preached. So why, then, can a suicidal person not "leave behind" other individuals when they themselves, and every other human, has experienced being left behind at one point in their own lives?