Earlier quoted context omitted.
Because reports have him receiving information either directly or indirectly from the Russian government. And they have access to all sorts of sensitive information. John Podesta and Hillary Clinton's emails allegedly found its way to Wikileaks through this path.
From what I've researched, the Hillary emails were most likely from an internal source.
And as has been pointed out the Podesta emails were taken through a phishing email which was sent to thousands of people (so it wasn't spear phishing) and which he foolishly fell for.