I suppose the question should be: why is Islam so much better than Christianity? After all it's growing while Christianity shrinks.
I don't think you can use terms such as 'better' which imply a superiority of one religion over another and it's been clearly demonstrated over centuries that leads to intolerance and discrimination.
All three mid-eastern religions trace their common roots back to Abraham and have much more in common because of that. The disagreements arise through differences of interpretation. The decline in Christian faith has been a gradual process and cannot be clearly discerned from a single event, like the Dissolution of the Monasteries, but post-war attitudes have brought the C. of E. to a point where questions of faith have become insoluble with matters of personal freedom: the right to question dogma has been much more prevalent in this country because a less Orthodox faith has been prescribed while Orthodox Islam has suffocated such questions in the Mid-east, leading them to stagnate and fall behind in matters of moral relativism and scientific advancement...(Bertie Russell explains it much more eloquently)
British Muslims face few restrictions on their right to question matters of faith and enjoy much greater political freedom than their native land allows, so their faith remains a bedrock in the community to bind them together where political adherence would bring about less cohesion. The argument between Sunni and Shi'a communities will always remain, but so does the difference between Catholic and Protestant. Not so much of a problem in England but the Irish still take it quite seriously.
The trend is an alarming parallel between people who profess to a Christian faith and those who do not advocate any faith at all - on the BBC last night, it showed the trend is almost a mirror image of the fall in faith and the rise in atheism. This line will appear to cross by the mid-century and i see nothing to prevent this happening. In fact it would appear to be inevitable.
After the reign of King Charles, his son will face a huge challenge to make any claim to be head of a dying Church while calls for the abolition of the Monarchy grow louder with each passing generation. By the end of this century there's a very real probability that this country will be broken up into its constituent countries and a secular, republican England will become a political reality.
I'm happy to say i won't be around to see that.