Thedudeabides explained about the lack of a ramped barrel in most 1911s. So why not use a ramped barrel? First, some manufacturers and/or shooters think it is more difficult (read "more expensive") to get the geometry correct on a ramped 1911 barrel for feeding purposes. If you try to split your production line into ramped vs. non-ramped barrels, this will increase costs.
BTW, Underwood does have one 10mm load for the Delta Elite and it is only 60 fps slower than the other loads in the same bullet weight. http://www.underwoodammo.com/deltael...ntboxof50.aspx