Could you clarify a little for the benefit of whoever might answer? Are you asking...

1. Why Moslem nations are "less modernised" than Christian ones?
2. Why religious law is still extensively employed in some Moslem nations?
3. Why Sharia Law is "less modernised" than the (mainly secular) legal systems of the West?

Or something else? The equivalent of Sharia Law in Christianity would be Mosaic Law, which is not really the basis for any Western legal system in the West, although of course some legislators still invoke it, e.g. in anti-holosexuality laws. The other possible comparison would be with Talmudic law, but that's a somewhat peculiar development obviously not representative of "Western" society at large.