I would guess that Sony doesn't yet have the rights to put dictionaries for many markets on its ereaders, and in some cases the content of the dictionaries may have yet to be digitized. In many places publishers remain reluctant to publish e-books, of course, and I'm sure dictionary publishers are no exception. Once the e-book markets take off, the dictionaries will probably follow.
In any case, it's a positive sign that at least one line of ereaders now provides a good selection of translation dictionaries.
I wonder how much Oxford University Press gets paid for every Kindle and Sony Reader sold!