Like all things, follow the money. Illegals are big business to a bureaucracy. Whole lot of budgets get funded "fighting" this problem.
Not to mention what the congress people have to gain, whether that be more power, relected or wealth or all 3.
It is insane to me that anyone thinks its a good idea to let illegals vote.
On the whole though, I don't think either side wants to fix the immigration problem or real work would have been done to get a solution.