Omprehensive immigration reform: Why the hold up?

Reform advocates are determined to keep the immigration debate alive, vowing to continue pressing lawmakers to pass immigration reform, if not this year, then next. But what is really at stake, are issues of the economy more important? or is it congress with the republicans eager to pass a bill on immigration and democrats stalling the process? What are the disadvantages of migration, the lack of immigrants, the history behind it all. And how about illegal immigrants already in America? etc…

I am trying to come up with a neatly worded easy that is engaging and with a powerful introduction. I am not on any side although I want to see the issue of illegal immigrants solved, either by giving them permanent or temp stay.