Because religious fools want every opportunity to control every aspect of everyone's life whenever possible, and that includes their death and even after your death. Cruelty is the point. No reason is good enough for them. They take away your right to an abortion for the same reason. Not because they care about lives, but because they want to cause suffering.
For those who aren't trying to control everyone, it's likely out of fear. It hurts to lose someone you care about. They're so blinded by terror they can't bear to open their eyes and realize how they are harming those around who are suffering and need a humane way out.
In a nutshell it's about power, control, and fear.