The main reason is that radical Islam has been allowed to flourish in a very real and visible way in London. As Robert Wistrich explained recently in Azure, in the context of a frightening article on British anti-Semitism:
In the past decade, the United Kingdom’s undisputed political, economic, and cultural center has also become a major world center of political Islam and anti-Semitic, anti-Israel, and anti-American activism. Through its Arabic-language newspapers, magazines, and publishing houses, not to mention its flourishing network of bookshops, mosques, and community centers, radical Islam has taken full advantage of what British democracy has to offer for its anti-Western goals, reaping the benefits of London’s significance as a hub of global finance, electronic media, and mass communications technology. The effect of this with regard to anti-Semitism and virulent anti-Zionism has therefore been quite different from that found elsewhere in Europe: Although Britain’s Muslim population of about 1.5 million is only a quarter of that of France, the growing influence of London’s Muslims has given the most inflammatory of ideas a greater legitimacy in the capital’s political and cultural discourse than they enjoy virtually anywhere else.It may take weeks to discover, but I am curious to see whether the perpetrators of this attack (one of whom, the detonator of the bomb on the bus, is currently rumored to have been a suicide bomber) are British-born, British residents or foreigners. Whatever the answer, I don't think there will be a backlash against regular Muslims; the Brits are far too stoic and sensible for that, and most people work with or live near Muslims, and can differentiate between radicals and everyone else. Multiculturalism is a source of London's current prosperity and rich cultural life and no one wants to change that. The answer, however, will have a tremendous impact on whether radical Islamists and radical networks in London will continue to be tolerated or not.