Because no matter how "moral" we think we are we still can not meet the minimum requirements that God demands of us. Your judgment will not consist of a set of big scales where God weighs the Good against the Bad. One bad deed, one Sin is enough to earn us Hell, no matter what Good we think we have done. This is why we can not earn salvation through religion or morality. Salvation is a gift. one we must accept through/in love.
Let's say a poor man loves a woman more than any man has before. He can not buy her nice things, but he still finds away to give all that he has from is heart. If one where to catalog his "works" the list might look something like:
hand picked wild flowers from the side of the road,
1/2 of his sandwich
a re-purposed greeting card with the mother's day message scratched out with a new message written inside.
an engagement ring from a gum ball machine.
For a woman looking for true love these items, these "works" mean the world to her. Not because of their intrinsic value, but because of the reason these gifts were given.
Now take these same gifts/works and attached to someone simply looking for a pass or a good time. what do you think they will mean to the woman looking for true love?
Do you think the acceptance or denial is in the gift/work, or the gift giver/ worker's intention? would you accept a shotty gift from someone looking to use you or take advantage of you?
Would you accept the same exact gift from some one who loves you with all that they have, but simply can not give anymore than what they are presenting?
Let's say you commissioned a professional artist and advanced Him thousands of dollars to do a portrait of you and your family in front of your house. let's say he comes to you with his finished work and says he went way over budget and needs several thousand dollars more for his finished work. When you see the work, you note a series multi colored crayola stick figures on a few paper plates with a Popsicle stick boarder. Would you give him the money he asked for. Why not? Now take that very same artwork and let it be by your son's hand. Now ask yourself what wouldn't you give him? I'd almost be willing to bet if he needed a heart to live your's would be available..
It's not just what you do that God looks at and judges, it why.
This is why Christ said all of the law and commandments of the prophets can be summed up with this:
Love the Lord God with all of your Heart, Mind, Spirit and Strength. Then Love your neighbor as yourself.
We can not earn our way into heaven because nothing we can do is worthy of Heaven.
We are Saved through faith not of works so no man can boast. However we do works because we love God with all of our being. With a love like that we can't, NOT show it. Just like the poor man will find a way to express his love, we being poor in spirit also will find away to show our love.. Just do not be so proud to think your wild flowers or gum ball machine ring earns you the right to salvation.