Why can't we follow the example of the Magna Carta in the Middle East?

Current Western policy in the Middle East involves a great deal of regime change. Whether we’re removing Hussein, Gaddhafi, or Assad, it’s all about changing the leaders of the country in question.

However none of these leaders can claim to be any more cruel and ruthless than King John of England. When King John was defeated in the baronial revolt, the barons forced him to sign the Magna Carta, but allowed him to remain as king. This was an important first step towards forming a constitutional monarchy, which later proved to be an extremely effective form of government, and over time most of the functions of the monarchy have passed to the parliament.

Regime change in the Middle East doesn’t appear to work. There isn’t a single example where a country has been decidedly better off for the removal of their dictator. Inevitably a power vacuum follows, leading to bloody sectarian conflict worse than the dictator’s violence. So why don’t the Western powers rather force these dictators to sign a bill of rights, enforced by the threat of regime change? Surely this would preserve the stability of the countries in question, while improving the political landscape for the citizens?