As one comment at Shorpy indicated, this tree wasn't "scrawny" in 1955 - this was a standard configuration in the era before trees were groomed in the field by mechanical pruners. The current generation of trees are remarkably dense and attractive, but the ornaments tend to rest on the branches rather than hang from them.
(From December 2008. Reposted because a tree that I transplanted into our woods 6-7 years ago is now this same size and has about this shape. In semi-shady woods I think vertical growth is more advantageous than thickening for young trees that want to capture more of the available light. I place a string of solar-powered Christmas lights on the tree "for the critters" (enough sunlight penetrates the woods in winter to power the setup, but the solar collector can't be stuck in the ground - it has to be a couple feet up to avoid being covered by snow).
Reposted again from 2011 to add the following:
The predictable reasons for the changes are discussed at the Washington Post.