The problem is that the government has become corrupt as such the secrets are especially bad. There is no fair trial from a government who picks the judges, the location, the jury pool, and the defendants lawyer and even worse the strategies of the defense (judges have to approve the defence).
I think one angle here though is the court of public opinion. In initial poles, most viewed what Snowden had done as breaking the law but didn't think of him as a traitor. He was a very popular figure and it would have been very problematic to put him on trial in the US. However, after getting associated with the Chinese, then Russians all on his way to try and get to somewhere like Ecuador or Cuba he's shifted the broader public opinion against him. Once he released it he should have best managed what his US based trial would look like - knowing that it was inevitable. The only way to avoid it forever was to become a pawn for some regime more despicable than the one he rebelled against.
