Because you eliminate entire groups of people without considering other attributes - if you met someone and only learned their religion some time later, you might find its not as important to you as you thought. If you rule it out before you can make that discovery , you never know what kind of people you have closed the door on.

Of course I'm someone who married the person who couldn't be more different in politics, personality and background - 17 years ago and going strong. So naturally my perspective here is biased - on the basis of criteria I thought were important to me at that age, I have no doubt I never would have considered her if I saw her profile and went by he facts and figures.