Because the Lord's supper was not the Passover.
So these words in 'bold" have nothing to do with the Apostles eating the passover with the Lord Jesus?:
"Then came the day of unleavened bread, when the passover must be killed. And he sent Peter and John, saying, Go and prepare us the passover, that we may eat. And they said unto him, Where wilt thou that we prepare? And he said unto them, Behold, when ye are entered into the city, there shall a man meet you, bearing a pitcher of water; follow him into the house where he entereth in. And ye shall say unto the goodman of the house, The Master saith unto thee, Where is the guestchamber, where I shall eat the passover with my disciples? And he shall shew you a large upper room furnished: there make ready. And they went, and found as he had said unto them: and they made ready the passover. And when the hour was come, he sat down, and the twelve apostles with him. And he said unto them, With desire I have desired to eat this passover with you before I suffer...And as they sat and did eat, Jesus said, Verily I say unto you, One of you which eateth with me shall betray me" (Lk.22:7-15;Mk.14:18).
If you ever had any credibility on this forum in the past then you no longer have any. It is absolutely ridiculous to argue that they did not eat the passover.
After eating Judas gets up and the disciples all think he is to go buy something for the passover the next night... John 13:39
No, they thought that he was going out to buy something for the "feast of unleavened bread," which came the day after the passover (Lev.23:5-6).
No lamb was mentioned no bitter herbs...and Wine was present none is commanded in exodus...and the bread broken was leavened bread not unleavened...Artos in greek not azymos...(this is still a schism between east and west churches)
No lamb mentioned? You have got to be out of your mind if you do not think that this is referring to the passover lamb:
"Then came the day of unleavened bread, when the passover must be killed" (Lk.2:27).
Also, notice that the day referred to here is the "day of unleavened bread" but despite that you say that they were not eating unleavened bread!
John 18:28 Has the authorities not wishing to enter a goyim court lest they be defiled to celebrate the Passover later that day...
So the last supper was the night before the next day when the lambs were killed and eaten that same day...
They were not worried about becoming defiled by eating the passover which was eaten in the evening because after the sun goes down they could not become ceremonially defiled:
"And when the sun is down, he shall be clean, and shall afterward eat of the holy things; because it is his food" (Lev.22:7).
The "passover" at John 18;28 is in regard to the passover offerings which were sacrificed during the day in the temple on the first day of the "feast of unleavened bread":
"And stand in the holy place...And Josiah gave to the people, of the flock, lambs and kids, all for the passover offerings...the rulers of the house of God, gave unto the priests for the passover offerings two thousand and six hundred small cattle, and three hundred oxen" (2 Chron.35:5,7,8).
The only animal which was eaten on the 14th in the evening were lambs. But here we see that cattle and oxen were passover offerings. Again, these passover offerings were in regard to the offerings of the first day of the feast of unleavened bread.
Of course if you cannot even understand that the Lord Jesus ate the passover meal as described at Luke 22:7-15 with His Apostles then the rest of this will be way, way, way above your head.