It wasn't at the time; it was a final meal for Jesus with his friends.
Of course it was hugely symbolic - Jesus, the Passover Lamb, was killed on the day that the Passover lambs were slaughtered. He was also the Lamb of God, John 1:29, 1 Peter 1:19, the Good Shepherd who lay down his life for the sheep, John 10:11 and the true vine, John 15:1.
But it was a Passover meal, not a service with a small piece of bread and a drop of wine/juice. And Jesus said "do THIS is memory of me" - when you have a Passover meal remember, not that you were rescued from slavery in Egypt, but that I have given my body and blood for your sins and rescued you from slavery to sin.
So if we're going to do it "properly", we too should have a meal - not a bit of bread/wafer and thimbleful of wine/juice in the middle of a service.
They were not respecting, or looking out for each other.
That is why Paul was annoyed with them - some got to the meal early and started eating straight away, without waiting for anyone else, 1 Corinthians 11:21. Some indulged so much they got drunk from drinking too much wine, while others went hungry, 1 Corinthians 11:21. He said that the people who had nothing were being humiliated by those who ate and drank what they wanted, 1 Corinthians 11:22.
He did not say anything about them not understanding the Jewish Passover.
Just like in the rest of the letter, the Corinthians were behaving in a divisive manner - in chapter 3 they boasted that they followed different leaders, in chapters 12+14 some of them boasted that they had "better" gifts of the Spirit.