Assange is alleged to have performed acts that could be construed as attempts to access classified information, beyond merely asking for it--specifically, it is alleged that he assisted Manning with attempting to acquire and decrypt classified documents. That is the heart of the issue--if Assange had merely accepted a document dump from Manning (as he generally did with most other sources), he wouldn't be facing 18 charges today.
"he wouldn't be facing 18 charges today" - I think it's pretty clear the US has been working on charging and imprisoning Assange since around the time of "Collateral Murder". They would have found something else!
Assamese already was facing a sealed indictment on other charges related to the Manning dump, charges that the Obama administration carefully drew up to avoid any 1st amendment implications. The charges being discussed here, being brought by the Trump administration, are the charges with first amendment implications.