I think it's because we are social creatures. Either people CTB and have people to mourn them. Or- they are alone and people feel sad for them because they were alone. I think some of it is this idea of missed potential- probably deluded thinking because obviously- that person's life was shit and they had no hope of it improving. Mainly though, I think it's to do with the effect it has on those left behind.
It can be a big deal to those left behind. Has anyone close to you died? I'd lost three close family members by the age of 10 through natural causes and honestly- it was devastating. The problem with suicide is the pain is deliberately inflicted early on them. If they haven't considered suicide themselves, they may not be able to get their heads around it. Plus, they may feel guilt that they couldn't help or, didn't do enough to help that person. I suppose it's the biggest form of rejection too. Not only are we making the choice to leave them, we are doing it knowing they will likely suffer. I think people sometimes feel angry at those who CTB. When they can't do that, they probably start deflecting it onto places like this. I think- on an individual basis, we can personally not have a big deal with it- but- we can't change how other people will experience it.