Why is the Bishop of Salisbury banned from the House of Lords?

The Bishop of Salisbury is not banned from the House of Lords; rather, he is one of the Lords Spiritual, which includes bishops from the Church of England who have a seat in the House of Lords. However, the number of bishops allowed to sit is limited, and the Bishop of Salisbury is not one of the 26 bishops currently entitled to sit, as only certain bishops are appointed to represent the Church of England in the House of Lords.