Because the evidence suggests they simply haven't!
They have no intention of intergrating. They form there own ghettos and for me it really is 50/50 whether or not they come into contact and take on board extremist beliefs. One of the major mosques in london was found to be preaching hate to thousands of them. God knows how much more is going on. The minority is growing simple as.
Funny that, could have sworn i know plenty of Muslim people who contribute a lot more to society than plenty of non-muslim people do.
By your logic white Irish Catholics have no intention of integrating with British society either. Look at the IRA.