I suspect it is more of an historical coincidence that for a long
time, most fantasy was either set in low-tech worlds, or in settings
in which technology played little overt part in development of the
story, while most science fiction had no fantasy element. One major
detail seems to be that many of the major early shapers of the field
were science and technology enthusiasts (Verne, Wells, Gernsback).
Once that restriction is lifted, many alternatives become possible:
Lucas-style science fantasy is one of them, but there are a number of
stories which more nearly resemble traditional magical fantasy which
have substantial hard-science underpinnings. I remember a
conversation with Katherine Kurtz, for example, in which she described
what the genes were that controlled the ability to acquire magical
power in her "Deryni" series, and how they were inherited, and how
that affected the plot development of her first several books.
-- Jay Freeman