Why space shuttle roll onto back during launch?

There are many reasons that the Space Shuttle performs a roll and pitch maneuver shortly after liftoff. The first is azimuth, or heading. When the crawler leaves the Shuttle on the Mobile Launch Platform at the pad, the cargo bay is facing roughly southwest. This is the most direct path from the Vehicle Assembly Building to the launch pad. For the cargo bay to face east, the crawler would have to execute a 180-degree turn, either spinning in place (not so safe for the Shuttle stack, and destructive to the crawlerway) or making a large loop near the pad (excess infrastructure). Long story short, the easiest way to spin the Shuttle is while it's flying, so the roll gets it pointed in the right direction, headed toward the correct orbital inclination, whether Hubble or ISS. The reason it's also called a pitch maneuver is because on the ground, the external tank is pointed vertically, but since the majority of the thrust in a rocket ascent to Low Earth Orbit must be oriented horizontally, the vehicle must be pitched over. The initial pitch maneuver, achieved through thrust vectoring via engine nozzle gimballing (primarily the SRBs, since at their distance from the center of gravity and with their superior thrust, they have greater control authority), puts the vehicle on course for a gravity turn maneuver that transitions it slowly from vertical to horizontal during ascent. The roll maneuver could be much shorter if the Shuttle flew with the Orbiter on top of the tank. So why does it roll nearly 180 degrees? First, because the angle of the Space Shuttle Main Engines relative to the vertical axis of the vehicle. Though their thrust vector is aimed at the center of gravity of the entire stack, the changing mass distribution due to the draining of the external tank makes them better suited to assisting the gravity turn if placed beneath the ET. Second, radio frequency communication between the antennas on the Shuttle and the ground is better when the signals do not have to pass through the external tank, as they would if the Shuttle were on top. Third, a heads down position allows the Shuttle to fly at a slight angle of attack, meaning the tip of the external tank plows ahead through the atmosphere as the vehicle passes through supersonic and hypersonic speeds, leaving the fragile Orbiter wings and tiles in its supersonic shadow, away from the strong aerodynamic forces that might otherwise damage it. Not to mention it's more fun for the astronauts to be able to look out the windows as the (upside-down) Earth during ascent, rather than the slow transition from blue sky (during day launches) to black space. As an interesting note, the Shuttle rolls back to Orbiter-on-top about five and half minutes into the roughly eight-minute ascent to prepare for ET separation. Naturally, the External Tank should fall toward the ground rather than being jettisoned higher into space. The "delta Z" maneuver at ET separation puts distance between the Orbiter and the ET, and the exhaust plume helps push the ET toward earth and cause it to tumble so that it mostly burns up in the atmosphere before plunging into the Indian Ocean. Then, finally, one more roll maneuver puts the Orbiter back into a heads-down position so the astronauts can look out the overhead windows and watch the world go by (and also so the communication antennas are once again facing the surface of the planet).