Remember he plead guilty to multiple crimes. Crimes he had taken oathes to uphold multiple times.
this isn't exactly true. the only crime he plead guilty to without stipulation was "wrongfully classifying stored information", which carries a sentence of two years.
the main guilty plea he offered to the prosecution was a limited version of the espionage act. in case people aren't already familiar, this is the law that the obama administration has used to charge twice as many americans as every other president before him combined.
anyway, manning was charged on seven counts of the espionage act. to be found guilty under the espionage act, they have to prove six things. manning plead guilty to a limited version of the charge because there was no way in hell he was going accept two of those six things.
the two conditions he definitely did not plead guilty to are at the very heart of the case, and in fact are all anybody in the public or the media ever bother bringing up.
a) that his leak was directly related to national defense
b) that he had reason to believe that his leak could injure the united states or benefit some other nation
bear in mind, the difference between the sentence of the full charge and the limited charge isn't beans. it's actually a difference of 56 years. and that's just the espionage charges, he plead for lesser counts of other offenses too.