Because illegal immigration has very little support among Americans. Many of those in favor of CIR are doing so because of practicality, little more. I don't support deporting 15 million individuals solely because I don't think it can be done and I don't believe that fantasy is a good basis for a political program. Most Americans were quite upset with the rallies a while back, with a lot of people who snuck into the country, waved their own flag (not America's) and then demanded citizenship and legal rights.
As a legal immigrant, it concerns me to get lumped in with that crowd in the eyes of most Americans. 99% of the time you see "immigrant" prefaced by "illegal", and for legal immigrants it means that our valid concerns (decent processing times, qualifications based on merit instead of relatives) get pushed back. I don't want to see that further reinforced by making common cause with lawbreakers.
I support some semblance of amnesty to illegal immigrants. But I also favor stricter border checks, much stricter workplace enforcement and a day-for-day penalty when applying for citizenship. If you lived here 10 years illegally, you need to wait an extra 10 years before you get to become a citizen.