Why is high-level military commanders using a private email server not a similarly big deal to a Secretary of State or First Daughter/Advisor to the President?
Because we don't vote for military commanders, and military commanders contribute little to nothing to political alliances outside the military, so there is no incentive for talking heads or Congress to notice it.
There's also a lot more of them than Secretaries of State and First Daughters, and the people that Congress and talking heads manipulate, us, couldn't sustain our interest past the second or third incident.
Props to Gmail :).