Some places will not and cannot get faster DSL in the US. The speed is physically limited by the construction of the phone lines; since the US was in the lead with infrastructure when phone lines were initially installed, many of the lines in the US are still very old, and typically made with aluminum wires (cheap at the time of phone line installation in the US). The line construction and physical distance add up to limit available speeds since you cannot pump that much bandwidth through poor lines over long distances.
Japan, on the other hand, is a much smaller and more densely-packed country; in large cities, you may be blocks away from the phone switches, and the infrastructure is likely much newer (probably post-WWII) and has nicer copper lines (someone knowledgeable about Japan may correct this).
The US will always be behind due to the cost of replacing and updating infrastructure and the pure distances involved - move to a big city if you really want faster internet!