Apparently you haven't followed the ET's tortured history. Originally the ET was built pretty stout - for a structure akin to two giant aluminum balloons stacked upon each other.
They originally painted the ET white to provide some additional thermal protection and match the orbiter, but they later skipped the paint job to save hundreds of pounds of weight.
Then when they decided to bring the Russians into the space station project, they raised the orbital plane of the station to allow the Russians to reach the station from their higher latitude launch range at Balkinour. This meant that the shuttle would have to expend more fuel plane changing to reach that higher inclination orbital plane. Not much could be offloaded from the orbiter (actually they needed to add extra cargo capacity to the orbiter for station), so the ET had to go on an even stricter diet. It was decided to make the ET out of Lithium-Aluminum alloy to lighten it up thousands of pounds. The only problem is that Lithium-Aluminum alloy is tricky to weld and fabricate. It tends to cause stress fractures, which weaken the metal. Eventually NASA/LockMart figured it out (the Russians had been building aircraft out of Lithium-Aluminum alloy for a decade) and they started flying the ET-lite.
Of course all of these post-Columbia safety fixes has added thousands of pounds of extra weight on both the ET and the orbiter. This extra baggage means that much less is left to carry people and cargo up to the space station. NASA desperately wanted to do this "extra-safe" shuttle flight perfect to prove to Washington that every thing is OK now - and then go back to thinning out much of the "extra-safe" dead weight to start restocking and continue building the space station.
Then falling foam happened - again!
NASA is in a tight spot now in so many ways. Merely fixing the ET foam is only a minor (but visible) part of what is going to happen to NASA in the near future.
Washington is not happy. The media is already writing NASA's obituary.
Thanks