Mara Salvatrucha operates in those places because Salvadorans are there, but i think its also inaccurate to say they operate in wealthy areas. Long island and the dc metro have large pockets of poor areas within the rich ones.
even Hempstead has a MHI higher than the median.
A slightly higher median income compared to the nation, doesn't mean much when you are in a much higher COL area. Also keep in mind there are different sections of Hempstead. The Cathedrial Gardens section for example is generally an upper middle class area and the area looks quite a bit like Garden City.