Because anti-discrimination laws around criminal records will work about as well as anti-discrimination laws regarding all the other protected classes (race, gender, age, etc.). Companies will very quickly figure out how to work around them, and will find ways to reject candidates for being a member of said protected class without making it apparent that this is the true cause for rejection.
If a company gets 90% through the hiring process, runs a background check, and then immediately says "no", then you sue them.
If this law passes, that will happen about as often as a company telling you to your face, "We're not hiring you because you're {black| a woman| over 40, etc}." What will probably happen is that companies will move the background check step ahead of the interview step. Then, after they've done the background check and had the interview, they can tell you, "Well, sorry, we've decided to go with a different applicant," just as they do today.