One reason is that,illegal immigrants may lower the wages and
reduces overall cost of production.This comes from the fact that,if
there are more illegal immigrants in the country,there will be more
immigrants looking for job because most of the illegal immigrants
prefer to work.This will lead to competition among low skilled
workers,which in turn reduces the wages of low skilled workers.And
if there is an increase in unskilled legal migrants,it will offset
the technological developments and may reduce the country's
economic international competition.