I've certainly wondered why. This is at least one possible explanation.
"Hillary Clinton aide Bryan Pagliano’s attorney said last week that his client would not cooperate with the FBI and would exercise his Fifth Amendment rights. What does he have to hide?
In fact, he may have already hidden information that will get him into a pile of legal trouble if he admits it.
It was revealed today by the Washington Post that Pagliano was paid to run Hillary’s private server — out of Hillary Clinton’s own pocket. She paid Pagliano an initial sum of $5,000 to set up the server. Pagliano dutifully reported the sum on his financial disclosure form in 2008.
But campaign officials now tell the WaPo that Clinton continued to pay Pagliano to maintain the server until Hillary left the State Department in 2013. The paper could find no record of Pagliano reporting any income from Clinton on his disclosure forms during the intervening years, and the State Department says they had “found no evidence” of Pagliano earning outside income.
Filing a false financial disclosure form is a felony — reason enough for Pagliano to plead the 5th.
[snip]
Another possible area of criminal exposure for Pagliano is that he wasn’t cleared to have access to classified information. If true, by granting him access, Hillary Clinton may be criminally liable as well.
Pagliano was an IT expert employed by the State Department. One can imagine several areas of overlap that would represent a conflict of interest.
In short, Mr. Pagliano is in deep trouble with the law and if the Clinton machine is looking for a fall guy, he would seem to fit the bill perfectly."
http://pjmedia.com/tatler/2015/09/05/is-this-why-clinton-aide-bryan-pagliano-is-taking-the-5th/