I think that Paul is on the right track. Also, the early diamond frame, with the sloping top tube, enabled a single frame size to fit more riders, by adjustment of the seat tube. As “scorching”—riding as fast as possible—became the craze, making the racing riding position popular, the horizontal top tube made a better aesthetic, and perhaps functional, fit.
Interestingly, although the diamond frame with horizontal top tube was dominant from 1895 to recent times, during the past twenty years, the sloping top tube has again become popular, sometimes even combined with the short wheelbase that was typical of the early 1890s.
In 2016, I purchased a Priority bicycle for every-day riding, and it is remarkable how much it resembles an early safety, especially once I added the “hairpin” saddle and lamp bracket. Note the similarity of its geometry to the 1893 Featherstone Prince that is with Craig Allen for restoration.