Speaking as the head of the IT group for Sony France, Lang told Clubic that Sony had tried a frontlight before (in 2008, to be exact) and that they believe it took away from the reading experience. Here is her explanation, via Google Translate:
We believe that the integration of lighting from the sides is not necessarily helpful and may even degrade the reading experience. It moves away from the traditional reading experience on paper. By cons, we believe it can be useful to have a cover that integrates a light that can guide the reader as he wishes. But we do not seek to incorporate lighting directly into the hardware of the reader.
Lang goes on to add that Sony has conducted studies from time to time, and while there have been requests for a frontlight it's never been anyone's minimum requirement.
I happen to agree with Lang in that I don't like a frontlight, but it certainly seems to be popular now. I would think having the frontlight would be preferable to giving up on potential sales.
On the other hand, Sony's continued presence in ebooks has puzzled me for some months now. I'm beginning to wonder if perhaps they are having more success than it might appear from the outside. Maybe current sales of the Sony Reader meets Sony's expectations. Unfortunately for inquisitive busybodies like me, only Sony can answer that. And that's not the type of info that is usually shared.