Mainly because it does upset most people when someone they love dies. If they didn't understand the extent that the person was suffering too- then, they'll see it as a tragedy for that person as well as themselves. As in- they could have gotten better but now, they'll never get the chance. Plus- there's the whole mental health stigma.
They probably don't want to believe that their loved one- in their right mind, chose to reject their life, chose to reject them and chose to kill themselves- knowing how much pain it would cause them.
If it's a parent- they surely have to acknowledge that they gave that child a life they hated to such an extreme that they were willing to risk all that pain and danger to leave it. Whether that was through nature- simply birthing them into this world with those genes. Nurture- maybe things happened in their childhood that they could have handled better or- a mixture of the two. Either way- I imagine that might be a horrible amount of guilt to carry. Personally, if I had a child that I found out was suicidal- it would break my heart- hence- I haven't had children. Why would I risk putting either of us through that?
But anyway- I'm sure it's much more comfortable to believe they weren't in their right mind. They were mentally ill when they made that decision. Even better- they were 'vulnerable' and happened to come across evil websites on the internet that coerced them into doing it. I think you have to bear in mind- these 'normies' think we're all crazy! Would you support a decision made by someone you truly believed to be mentally incompetent? I'm not saying we are of course- I'm saying- they think we are. In fact- they are probably convinced that we are.
If of course a person is unfortunate enough to have no one that cares about them, I still think people project. So they- in this case wrongly- still imagine there will be people out there devastated by the loss. I think the thing is- maybe suicidal people put themselves in the suicidal person's shoes and the 'normies' put themselves in the shoes of the people that survive them- think what this will do to the people who love you. Probably because they maybe can't get their head around suicide. I think it's almost at a criminal level in some people's minds.