Because a lot of people think that the information brought to light by Manning was instrumental in helping the world, rather than hurting the world. And that whistleblowing involves risking yourself and your safety for the betterment of society. Manning made data public that did not make her life better, and did it because it was the right thing to do despite the massive (and well founded risk).
There is a secondary argument that her treatment by the US Government has verged on torture, and definately crossed the line of cruel and unusual.
"Don't do the crime if you can't serve the time."
Manning isn't asking Obama for a pardon. US Citizens are.