Because not many other countries allow them such unfettered access I guess. There is also the fact that nobody can die of starvation in the UK, unlike some of the countries without a welfare system which they may come from.
I can't remember the exact figures, but it was something like only the UK and one other country allowed free movement of labour from the new EEC accession countries when they joined last January, [or was it the one before?] so that is the main reason that we have received such a large influx from Poland et al.
Giok