Cynical politics and economics play a role here. Republicans don't want formerly illegal immigrants voting anytime soon, since poor, low-skilled households aren't going to produce many GOP voters for a generation or so. And business doesn't care about citizenship one way or the other, as long as it gets its cheap labor. That's why employers support the indentured-servitude-style guest-worker program in the bill.
It is corrosive of American civic ideals to have widespread violation of the law and a class of people who aren't fully a part of American society. This bill -- which is neither soft nor tough enough -- will quickly return us to exactly that position. People who have absconded from deportation orders and aren't automatically eligible for the Z visas (some 600,000 people), illegals who have come here since January 2007 and are ineligible, and illegals who won't bother with the rigmarole of getting a real Z visa will form the basis of another large illegal population.
This is why a rational approach to immigration must start with enforcement. Only when enforcement is real would it be possible to give an amnesty to those illegals still here without repeating the experience of the past 20 years -- an ever-growing illegal population after an amnesty -- all over again. With a viable enforcement regime in place, illegals still here could get a path to citizenship more generous than the Know-Nothing version in the deeply flawed Senate bill.