originally -thanksgiving had nothing to do with turkey, wild turkeys take a long time to cook to make them tender , so initially it involved other foods like deer and ducks and such.
turkeys for thanksgiving evolved because of the farm raised turkeys which had alot more white meat on them and were easier to cook and were cheap at the time and as imlonely said you could feed a lot of people with a turkey.
so if the turkey had never been domesticated we may be celebrating with a goose or a duck.
i personally celebrate with deer meat in the slow cooker (as well as a turkey).
I think because it was easy to get them and the bird is big enough to feed everyone
because the it was a favorite of the Native Americans. it was easier to make.