Because the next time might be the time that it works. Because you're not finished yet.
Because the mission is big enough.
Because it's the right thing to do.
Because you believe it's worth the cost.
Because it's worth fighting for.
Because people are counting on you.
Because your example inspires other to try.
Because you want more for yourself.
The question isn't "why" but "will you?"
That's the only thing that really matters.