Because the groundwork for this fiasco was laid down by President Bush. His plan is a fiasco and the idiots in Congress will make it worst.
Considering that AgJobs was introduced in 03 and negotiations on that bill began 3 years prior to that, you would say that that even precedes Bush's presidency. In fact, if Gore had defeated Bush in 2000, AgJobs would still be the bill that everyone supports.
Let's look at it another way.
Although there was some immigration legislation in the 90s, it was mostly minor changes and adjustments. You have to go back to 1990 to find any significant changes in numbers. And back to 1986 to find anything dealing with citizenship.
So you see, Congress was due to make some changes, irregardless of what Bush may or may not want; irregardless of what Gore may or may not have wanted.