Socialism?
I think that term doesn't scare most folks like it did maybe 20 years ago.

Too old?
Bernie is pretty energetic and doing fine with the rigors of the campaign trail. Hillary isn't a spring chicken herself.

Mass Appeal?
Bernie garners the largest amount of interest as far as campaigning because his ideas resonate. The more people get to know him the better he does.

Political Record?
He currently possesses the most consistent record of all the candidates. Can work with both sides of the aisle.

Foreign Policy?
Doesn't prey on fears so I guess that works for the GOP and maybe Hillary to a small extent, but I think that the world at large is behind his policy as well as the soldiers... not sure about military brass

I could go on and on.

Clinton is a known quantity yet she still has new stuff popping up like whack a mole... what's up with that? Do we feel secure with that? I think you brought up before that the Feds digging in her activities could roll snake eyes.