Carter Page has a history of working with both the CIA and the FBI.
He has numerous Russian contacts.
He found his way into the Trump Campaign.
He was the target of the FISA warrants.
He has never been charged.
But, those FISA warrants opened the door for an expansion of targets within the Trump campaign.
Page was the target of the investigation. They used his alleged links to get warrants. But somebody gave the FBI his name and the story about Russian links. And another person (Stefan Halper) was trying to get information from Page and others who might have ties to the Trump campaign. If Page wasn’t charged, it could just be that they didn’t find anything to charge him with.