Snowden is afraid to go to court because, unfortunately, he has no viable defense to the charges. Snowden's defense lawyers want to make a so-called "public interest" defense, but neither the DoJ nor the courts are likely to permit such a defense. (Though, I'm unsure if a court might allow it if the prosecution failed to challenge it.) This is a well known point of contention publicly described by both Snowden and his lawyers for years now.

This is why people, including Snowden himself, can have such a high degree of confidence in the outcome of any trial. For the situation to change, Congress probably needs to modify the law.