#1 Many would rather not become citizens. There are a lot of benefits to not being a citizen.
#2 We're about to legalize a lot of unskilled workers. Do we really want to do that? Unskilled workers who would have to get paid minimum wage, who rarely are able to speak English. A lot of them would just end up using our social programs.
#3 Doesn't this just open our borders for anyone to come in? Most countries are extremely selective of who they bring in. Usually only someone who can contribute can come in.