There's a lot of reasons why D20 just doesn't fit with Shadowrun. The
probability curves are completely different and it'd be a nightmare to
adapt every number and test. They're just really incompatible.
But apart from numbers it's a major feel thing. D20 feels like the hand
of fate being modified by godly abilities. It's why it suits your
traditional high fantasy settings with super-powered characters.
The SR dice system is about your every bit of skill being brought
together to overcome a challenge. I've never understood the complaint
of "too many dice!" (I really think it's not a real issue anyone has,
just one they're taught to have.) It suits being a scrappy underdog in
a deadly world. You feel week if you're rolling few dice. You feel
strong when you're rolling fistfuls.