The information comes from Apple designer Christopher Stringer who took the stand during the Apple/Samsung trial.
Satzger explains the reason for not going with the curved glass:
The technology in shaping the glass, the cost relative to shaping the glass at the time, and some of the design features of this specific shape were not liked.
The technology at the time had a lot to do with it. The qualities of the glass at the time had a lot to do with it. These are models -- I'm trying to remember a time frame -- that were before gorilla glass and before a lot of the other factors.
During his questioning Samsung tried to get Satzger to say that the design of the iPhone was determined by drop tests.
Samsung: "The reason why the iPhone designs took various forms along the way was in response to drop test results, among other things; right?"
Stringer: "I can tell you quite plainly that this shape is not determined as a result of drop tests."