Because he doesn't want to cloud of suspicion hanging over him for the rest of his career.
Realistically, there's only so much he can do on that front. It's not like the people who are inclined to be suspicious of someone for PED use need any actual proof to have it. Look at the "questions" about Bautista or what's happened with Jeff Bagwell in the HOF votes.
Braun will take more drug tests throughout his career. If he's doing anything that can be caught he'll be caught. If he's clean, he won't be. If there are people who are going to see that cloud hanging over his head despite whatever will happen in subsequent tests then I'm inclined to think nothing Braun could have done would have lifted it anyway.