Because it means Shariah law, a political system that privileges Islam over all other religions. Because it means putting a system of law in that reinforces religious edicts over everyone, regardless of faith and regard for human rights.
So you get results like Aceh province caning an elderly Christian woman for selling alcohol, Shariah patrols punishing any woman for not veiling. You get courts that count a woman or Christian's testimony as half that of a man, as well as devaluing them in inheritance cases.
Child marriages, spousal abuse, marital rape and denying a woman support while failing to divorce her all become legal when Islamic religious rules take effect.