Why? because just he's a Manchester MP? As mentioned before, the suggestion that no one outside the south east wants LHR expansion is risible.
But Stringer could have been more vociferous in promoting the idea that by encouraging growth at regional airports and allowing them to satisfy more of their own demand on direct services, this can help the situation in the SE. This would free up some, albeit small, amounts of capacity for people who really need to go to or through London's Airports. This will become increasingly important as whatever happens in the SE won't add much extra capacity in the short term.
Stringer used to be a massive supporter of MAN but seems to be much less enthusiastic nowadays. He was of course a big cheese on Manchester Council when it was the majority owner; I wonder whether the formation of MAG and the sale of some of it privately due to the acquisition of Stansted has been one of the factors in this apparent reluctance to bang MAN's drum as hard as he used to?