Can’t say what’d be the why for you, just what it is for me. My top three reasons (and the only ones I can think of off the top of my head) not to are:

1) Not wanting to bring the hurt on those who care about me (I know after I’m gone it won’t matter to me, but until then it does)
2) Fear of physical pain in the process and of the possibility of failure in the attempt
3) That I believe life truly is random which, though most of the time painful, also means I can’t know the future. By chance, they might come up with Awesome New Working Happiness Drug/Therapy tomorrow, and had I killed myself today, I’d miss trying it. Or I might save someone’s life; someone who does value theirs. Would’ve made it worth it I struggled until then, and I’m still as free to end it after that.

Only #1 would alone suffice, #2 and #3 I use just to supplement it. If I didn’t have anyone who mattered to me (and vice versa), #2 and #3 even put together probably wouldn’t keep me here.