There are migrant and there are migrants, and they are not all the same. But that said many migrant workers are coming from an area of of the world with economic difficulty, low wage, low cost, low prospects, and so have much lower expectations on what constitutes a fair deal than those born here. Consequently they are more prepared to agree to worse conditions and lower pay as for them it is seen as an improvement. I suspect this accounts for much of the issue. If there is a workforce willing to come here and undercut local expectations of what is fair, then they get the job. A simple divide and rule tactic on a global labour market.