Why is it easier for the prime minister of britian to pass legislation than it is for the u. s. president?

In theory then, the American President has much more power than the British Prime Minister - he is the commander-in-chief and has the power to issue executive orders which have the full force of law. However, the constitutional system of 'checks and balances' seriously circumscribes the power of the US President who often finds it really difficult to push legislation through Congress. By contrast, a British Prime Minister usually heads a government with a majority of seats in the House of Commons and the ability to pass almost any legislation that he wishes.