Because it is religious law, period. Any form of religious law that is made to govern a country is a bad idea.

That's because people cannot argue and discuss religious law. If you disagree, you get accuse of anti-Religion, or how dare you go against the will of (religion deity)?

Just look in the past, when the Church was so powerful. They make any laws they want. Bribery was rampant and the priests were corrupt. But people don't dare to speak up against the "holy people". If they do, they are labelled infidels, going against the will of God etc.

Even now, lots of people still put a lot of blind faith into religious people just because they are "religious folk". But still in papers you can read about the paedophilia scandal in the church. In the local news you can see some religious teacher having sex with underaged kids they are supposed to tutuor.

Especially in our society, how often you see an ustad/mufti gets condemned for giving out a stupid statement? People might say in private or in hush whispers, but few dare to do it because you know, he is religious/knows more about God etc. Compared that to a regular politician or NGO. People will bash him openly left and right for giving out stupid statements.

When a law is secular, it means it is created by humans and is not perfect, it can be discussed. Different people can discuss, amend or scrap it if they want. But when it comes to religious law, it cannot be challenged. It's because "God said so and God is perfect" - how do you even move from the premise. Never mind the fact that often these laws get twisted by religious folk, and as mentione few dare to challenge these religious folk.