A college friend who understands the shuttle far better than I explained that the integrity of Shuttle Program was compromised from almost from its inception. In an attempt to keep the budget in line with forecasts the designers were forced to add the solid-fuel, strap-on boosters to get the extra thrust needed. Solid fuel boosters have some benefits, but there are also some drawback -- like you can't "throttle" them. Plus, there is the real possibility of a catastrophic explosion if there are any voids in the solid fuel. When the Russians designed their shuttle they dispensed with the solid boosters.
Bottom line: NASA expanded the amount of risk that they considered "acceptable" in order to keep the bucks down.