Because we only have one political party,and the elites in both brances who run this party see personal profits and power coming from the invasion of immigrants. They don't give a damn what the sheeple who vote for them think.
Both major parties are ending up in the same spot now, but I see two different debates actually taking place.
In the Democratic party the ethnic politics coalitions and the lefties have pretty much won the debate. You don't hear any opposition to immigration anymore from the traditional blue-collar unions, at least not that I can recall lately.
There are a lot of business interests on the Republican side pushing immigration. And there is a libertarian argument made in favor of immigration. But I think we still have a debate going on in the GOP. Although at the moment the pro-immigration people have the upper hand.