I think that it's because of peoples delusional beliefs in regards to the cruel reality of existence. Many people live under this delusion that all life is worth living and that life has value. By seeing suicide as a valid and acceptable option they would have to come to terms with the fact that life could get that awful for them to the point of considering suicide and this would shatter their delusional worldview. I believe that wanting suicide can be perfectly rational in a world like this, wanting suicide is seeing the world for what it really is. Non suicidal people simply don't understand as they are not suicidal themselves, we all experience life differently after all.
I also think that euthanasia isn't legal because the society needs workers so therefore they need people to stay alive. There are so many people trapped in low quality lives who would likely choose suicide if that option was there for them to pass away in a peaceful and reliable way.
But I think it's unfair how animals can get euthanasia and we cannot. Dying would solve all problems in which there was never a need for in the first place. Non existence is ideal as it's the absence of all suffering.