short answer?
"buh killing people bad/ death bad"
more nuanced answer?
most people are terrified of death and can't imagine wanting to end their own lives, or can't imagine themselves in a situation where the best choice is to kill themselves. there's so much insistence that "life is a beautiful gift" and "it'll get better", and there's some truth to that. But not everyone has reached the lows in their lives that others have. Some have gone through the worst and just have such strong mental fortitude that they manage to preserve and find meaning in their lives.
Most of the pushback against the right to die movement is people who simply don't know what it's like to be with their backs up against the wall, with no support system or nothing that can change their circumstances. Or they might think that they have no right to end their own lives and neither do others, because if their god wanted it to be like that, he wouldn't have made them go through whatever they did.
Humans have a strong will to live, and most have a very black and white moral code, and though the species has evolved through empathy, we sometimes tend to lack the deeper understanding of how our fellow man may feel deep inside.
Simply put, a lot of them can't put themselves in our shoes because they've never been where we have. Imagine making someone run their first ever race in high heels when they haven't even jogged a mile in comfortable shoes before, let alone worn heels. They're gonna complain about how much it sucks and they hurt, and they're gonna fall a ton. They're gonna say "you can't do this, there's no way anyone ever does this. I bet they just quit or switch into heels at the last moment", but there's a race every year, where men run in heels (look it up, I'm serious. it's actually cool)
tl;Dr: life is too sacred to them, or they can't relate to having enough of a reason to die, or perhaps they're too cowardly to