Because, Corby, many of the non-native English speakers that are doing well usually have a better grasp of the Mother tongue than some white English children. They have such skills because their parents speak good English and, living in the UK, have decided to adopt it as their first language.
Large numbers of the children I am talking about come from homes where little if any English is spoken and in a number of these homes there is no intention whatsoever of adopting English as the first language. A lot of these people have been here for a generation or two but they have been encouraged to continue as if they were living elsewhere, becoming marginalised from the English speaking community and instead developing quite separate communities of their own. The other part of the problem is that some recent arrivals, especially from Eastern Europe, have been here a relatively short time and have had little opportunity to gain skills in English.
I have spent some time observing teaching (mainly primary) in some of the places I have mentioned and, believe me, it is a wonder in some schools that teachers manage to impart any knowledge at all. It is a huge problem that is costing enormous sums of money (by having to provide, effectively, interpretation and translation facilities in classrooms) and which is hampering the education of all pupils.