To put it simply: because our immigration non-policy of the last 40 years has destroyed any meaningful difference between "legal" and "illegal" as far as immigrants are concerned. It's an open secret here in Orange County that Bob Dornan was voted out of his congressional seat in 1996 by voter fraud involving illegals illegally voting.
Further, there was recently a report that all residents---no matter what the immigration "status"---are counted in determining congressional districts, distribution of taxpayer "goodies," etc. This creates "rotten boroughs" filled with illegal immigrants (e.g., Bob Dornan's old district) where the representatives of such districts wield as much power in our government as those from districts with few immigrants.
But mostly it's a self-fulfilling prophecy. The 1986 amnesty, "sanctuary" laws, corporate scofflaws, the general failure to enforce deportation orders, and now this "reform" proposal illustrate to the politician of average intelligence that as long as an immigrant gets here, no matter how he gets here, he's gonna stay here. And if he's going to stay here, he's gonna be a "constituent" of some politician, and he's gonna vote (legally or not) and want taxpayer services (entitled or not).
When the law doesn't get any respect, the lawbreaker always does. More respect in fact, than the chump who continues to obey the vapid law. That's why politicians are offering illegals bennies like in-state tuition rates, while out-of-state citizens have to pay "alien" rates.