The History of the T-shirt.
Here is a construction worker relaxing on a steel girder. In flicking through pictures of the early part of the 20th Century, you might have observed that it was only workers who wore vests or singlets. It would be difficult to work in shirts and waistcoats would have been possibly uncomfortable, removing them would be made more accessible to work. There could be other reasons; the clothes would have been expensive to replace. As seen in the next photo of a steelworker relaxing on the empire state building circa 1931.