Because the fact that they were Muslim is more telling than the fact that they were from Pakistan - because even if there are only precious few Pakistanis in the U.K. who aren't Muslim, it still deserves pointing out that aren't just "from another country," but "Muslim."
Imagine reading a report that a group of youths - "two Egyptians, one Sudanese, two Saudis, and three Indonesians" - had been arrested for carrying a bomb into an airport. And let's observe that they also had "religious leaflets" with them. According to your logic, it wouldn't be relevant to instead report that eight MUSLIMS had been attempting to smuggle a bomb into the airport (since virtually all citizens of those countries are Muslim), and that they had been carrying leaflets calling for a "worldwide djihad" (since that can almost be taken for granted, given the first item).
Regards,
Pakistani and Asian in UK don't (mostly) refer to immigrants from those countries. It refers to racial/ethnic groups, most of the members of which were born in UK.