The dagger has always been a thrust weapon. You are supposed to use the point rather than the edge. Double edge knives are genrally weaker, less sharp, and less versatile than single edge knives.
With a dagger the edge is bevelled through the centerline of the blade, and the centerline is the thickest part of the blade. With a single edge the bevel can run all the way back to the spine. Which not only gives you a better edge per given blade thickness, but also allows you to cut deeper before you get to the maximum thickness of blade.