Mike's mentioned this before, and as I recall, the basic problem is that they'd have to scan and OCR it themselves, as well as adding a number of rare characters to their font files.
Matthews is also still under copyright, and they don't expect the number of sales to be high enough to make it worthwhile.
I'd love a copy of Matthews on Pleco (I hate Wade-Gilles...), but I just don't see it happening for another 20 years or so (or whenever Matthews finally loses copyright and some extremely dedicated fans scans and OCRs it).