It really doesn't. Two-syllable names really don't, and few have them. Nicknames came about partially as a way to shorten three-syllable or longer names, so as not to have to go through the verbal gymnastics of saying longer names on a day-to-day basis, multiple times a day. It's counter-intuitive to come up with nicknames for two-syllable names that trip right off the tongue.
Heather is one of those names.
My older sister and myself have two-syllable names and neither one of us has ever gone by a nickname. My two younger sisters have three-syllable names and they both have always gone by a nickname. That's what comes intuitively.
But then I guess it does happen at times. Some Janices goes by
Jan, though I don't know why, as I would never want to go by that. And my sister
Linda, mother of
Heather, would be called
Linnie by her first mother-in-law, though my mother hated that as well as Hezzie. Seems like that family were compulsive nickname-givers.