Sharia law is a body of religious law that forms part of the Islamic tradition, based on the scriptures of Islam, particularly the Qur'an and Hadith. It's seen as the expression of God's command for Muslims.

Countries implement Sharia law to different degrees, ranging from strictly conservative applications shaping the entire legal system, to hybrid systems where Sharia informs certain parts of the legal code. Historically, Sharia existed alongside other normative systems.

As for why some countries with Sharia law might not fully adopt the legal systems of countries like the UK and US, there are complex historical, cultural, and political factors at play. Some may view Sharia as a more advanced or appropriate system for their specific context, or they may seek to maintain a distinct legal identity rooted in their religious and cultural heritage.