My understanding, from people who know people, is that Shadowrun sells well enough as it is. I guess FanPro didn't see the need to switch to d20 as it wouldn't boost sales that much.
That's sort of like how Alderac found out that the d20 system didn't bring in that many new players (I'm an exception to this).
What I would have liked to see is a dual-stat book, one that used Shadowrun rules, but gave a d20 variant as well. However, this does take more time and takes space away from books.
There are fan conversions out there, and unfortunately I can't seem to find one on the web anymore. Likewise, between d20 Modern, Urban Arcana, and OGL Cybernet, you can work up a d20 conversion.
I think we're going to start seeing a shift in the RPG industry away from d20 anyway, so it probably is just as well that Shadowrun remained a non-d20 system. *shrugs*