... because unlike every other Muslim leader in the UK he is quite explicit about what Islam means and what Islamists ultimately demand.
Take a look at this short video interview with him. It is a shame people in the UK do not understand that he is not an 'isolated extremist'. He simply says what all other Islamists believe, but try to stop you from knowing.
And I strongly suggest you read the latest brilliant article by Sultan Knish to prepare you to respond the next time you hear someone spouting about mistreatment of Muslims.