"For I received from the Lord what I also passed on to you: The Lord Jesus, on the night he was betrayed, took bread, and when he had given thanks, he broke it and said, 'This is my body, which is for you; do this in remembrance of me.' In the same way, after supper he took the cup, saying, 'This cup is the new covenant in my blood; do this, whenever you drink it, in remembrance of me.' For whenever you eat this bread and drink this cup, you proclaim the Lord's death until he comes." NIV translation
Paul tells them (and us) that they were not doing this because they were ignorant because he had shared with them the words of Jesus concerning the Lord's Supper. In this passage, we see that the purpose of the Lord's Supper is twofold. Observing it gets us to stop and remember what Jesus has done for mankind and it is also a witness to others. In many "churches" today it has been twisted around and many desperately take communion as if it is a chore required for salvation. As we see, the focus was not on us doing something to be saved but on remembering what Jesus did to save us.
"Therefore, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. A man ought to examine himself before he eats of the bread and drinks of the cup. For anyone who eats and drinks without recognizing the body of the Lord eats and drinks judgment on himself." NIV translation
What does Paul mean by an "unworthy manner"? We see from the instructions that Jesus gave concerning communion that it is to be done as we remember what He has done for us. They were coming together and eating like it was a regular meal and he warns them that they are bringing judgment on themselves.
"That is why many among you are weak and sick, and a number of you have fallen asleep. But if we judged ourselves, we would not come under judgment. But when we are judged by the Lord, we are disciplined so that we will not be condemned with the world." NIV translation
We see that the judgment that Paul speaks of is physical sickness and even dying physically. We might ask ourselves how this can be and we see that this judgment is actually discipline to get them back to remembering what Jesus has done.
"So then, my brothers, when you come together to eat, wait for each other. If anyone is hungry, he should eat at home, so that when you meet together it may not result in judgment. And when I come I will give further directions." NIV translation
Paul's final words on the subject of communion reminds them that it is not about getting food in the belly but remembering Christ. He assures them that when he visits them he will show them what he means by worthy communion.