One reason - highlighted by peter willets in his book The Pinch, is that Eastern European immigrants who come to cities temporarily without families are much more likely to tolerate 'under-housing' eg at least one bed in nearly every room of the house, no communal space etc. They put up with it because it's temporary.
In an area with high housing and transport costs, their willingness to under-house makes them far more able to accept low paid work than local people seeking a more lasting and settled arrangement.
As always, it comes back to Thatcher and the monetising of peoples' homes.