It's pretty simple... for about 30 years, the basic pattern used was what is called a "dartless" -- which works well for loose clothing and stretchy fabrics, The dart uptake (the part you sew out in a dart) was distributed to other areas of the garment, like a little into the shoulder seam, a little into the armhole, a little into the hemline.
When the styles switched to more body-conscious styles, darts or dart equivalents (like princess seams) came back.
Generally, the basic block (basic basic pattern) used for most clothes seems to switch about every 30 years.
In the early 1900s, avant garde clothing was "arty", like http://www.google.com/imgres?imgurl=http://www.metmuseum.org/toah/images/h2/h2_1995.28.6a.jpg&imgrefurl=http://www.metmuseum.org/toah/works-of-art/1995.28.6a&h=661&w=300&sz=49&tbnid=b6LxW3rKAkLncM:&tbnh=90&tbnw=41&prev=/search%3Fq%3Dfortuny%26tbm%3Disch%26tbo%3Du&zoom=1&q=fortuny&usg=__0cfC8gfurnr9UdUx-jdyX4yyy2g=&docid=ddqDu7OrIYZlTM&sa=X&ei=FFD6T9SNDtT2rAGf69yMCQ&ved=0CH4Q9QEwBQ&dur=530 Fortuny; that held through the 1920s: http://www.vintagetextile.com/images/1920%27s/6955.jpg and then in the 30's, you started seeing dart fitting patterns again creeping in. By the 1940s, darts and seams that are hidden darts were in full sway, and that continued up to the mid 60s, when "hippie clothes" and knits took over in a big way, and we ditched the bulletproof bras and girdles...
Just another swing of the fashion pendulum.