I’ve never thought it was anything but a matter of personal or family choice.
In my family of origin in New England, Thanksgiving dinner was usually about the same time as Sunday dinner—i.e., early afternoon, after church.
In my present household, my husband and I used to plan it for that same time of day when his parents were our guests because they wanted plenty of time to visit and still get home (a half hour’s drive) before dark. We’d get the turkey stuffed and ready the night before, and I’d get up at 7:00 a.m. and put it in the oven. We devoted all morning to preparing the rest of the meal, and we’d eat about 12:30 or 1:00 p.m.
In later years, when they were gone and it was just us two, our sons, and sometimes their friend or girlfriends, we gravitated toward an early evening dinner hour so we could do our preparations during the afternoon. We’ve also scaled back to make it a simpler meal, and we do as much as possible the day or two before.
One year the oven went off while the turkey was roasting, and nobody realized it until it was all cold. We restarted everything and had to guess at how much more time we needed. We didn’t eat until 11:00 that night, probably making us the last to dine on the continental U.S.
I think that when you’re the host you get to decide, and whatever you decide is right.