I guess she's just very comfortable in them and it's sort of become her trademark.
Because she beautiful, sexy and confident in wearing pant suits. As AH stated it's her signature trademark.
Because she's 67 and her legs are awful.
I heard she has a bladder bag on her leg due to inconintinence
What difference does it make? You wouldn't expect Trump to wear a skirt or kilt would you?
Could be another reason too.
Because the rest of us don't want to see her thighs.
She is of the older persuasion, and as some of us know, flatulence can become a bit of a problem as we grow old gracefully. There comes the time when there is no forewarning, it just put's itself out there.
However, Hilary is a Presidential Candidate in the spotlight and so she wears Fart filtration underwear, hence the Pant suits. Donald wears them too.
Why, not sure---but I am somehow strangely grateful.