It never was. Since antiquity it had various Semitic people with Akkadian and Aramaic influences surviving in the spoken dialect of the Shia majority, the Sunni minority speak a different Arabic dialect. Before Islam it was predominately Christian.

That said, the current Sunni monarchy conquered Bahrain in the 18th century. The majority are Shia Arab who are very sympathetic to Iran and don’t like hosting US troops. The last indigenous Shia rulers of Bahrain ruled on behalf of Iran.