I believe this was just a critical failure of some sort that occurred (for reasons as yet unknown) at a crucial time as the shuttle was in re-entry. A catastrophic failure at that time could not be recovered from.
Although I am not certain at this point what the cause was, clearly, there was a major problem along the left wing that resulted in all of those sensors going out and increased pressure on the landing gear tires, etc. Something bad went wrong during the most severe thermal stress on the vehicle, the results spread rapidly and led to the breakup of the vehicle.
Space exploration is, by default, dangerous and every one of those involved know it ... and accept that as a part of their life in pushing the frontier forward. From time to time, we are reminded how dangerous by such sad events as these.
I am impressed at the forthrightness of NASA at this time and believe that we will find out what the ultimate cause was soon.
It was a tragic day for those people and their familes ... for our nation and and for the program. But I believe we will rise above it and continue, with even more purpose.