To answer this question one has to first describe and provide reasons for the rules which govern the legislative powers of the House of Lords in comparison to those of the House of Commons.
To properly appreciate the rules one should determine the composition of the people who are subject to the rules first. Each member of the House of Commons represents a constituent of the UK and is voted for by that constituent, voting takes place once every term of parliament. Members of the House of Lords are selected not on a voting basis and are chosen from one of the two following methods. Hereditary members are those who inherit their status as from their family, non-hereditary members are chosen by the prime minister in recognition to their expertise to become life peers.
In order to fully understand the purpose which the rules that govern the legislative powers of the Lords are set to achieve and other relevant aspects one must look back in history and consider how the House of Lords has changed since its creation and the reasons behind those changes.
The process started during the reign of the Normans when king used to call the Magnum Concilium and the Curia Regis to discuss national affairs. The Greater council was made up of ecclesiastics, noblemen and representatives of the counties. Its main function was to approve taxes proposed by the Crown; it developed legislative powers as those who were taxed gained representation gradually.
The Greater council evolved over time. It split into the House of Lords and House of Commons in the Reign of Edward III in the 14th century. At that time, the House of Commons consisted of the shire and borough representatives and the House of Lords consisted of the senior clergy and the nobility.
From the 14th to 15th century, the House of Commons was far less powerfully than the House of Lords because of the affiliations of the prelates and aristocrats.
The first major decline of power of the Lords happened in the 15th century...