Because of the pattern laid out in Lev 23 concerning the sacrifices of the Passover lamb and the consequent offering of first fruits. Jesus is both offerings.
Lev 23:5 In the fourteenth day of the first month at evening is the LORD'S Passover.
Lev 23:6 And on the fifteenth day of the same month is the feast of unleavened bread unto the LORD: seven days ye must eat unleavened bread.
Lev 23:7 In the first day ye shall have an holy convocation: ye shall do no servile work therein.
Then on the day following the Sabbath, which would have been the 16th of Nissan, they were to bring the offering of first-fruits.
The death and resurrection of Jesus had to follow this precise pattern. Since, as scripture tells us, he was raised on the first day of the week, this means that day had to be the 16th day of Nissan. This means that the Sabbath was the 15th of Nissan and Friday was the 14th of Nissan. This was the day he was slain and buried.
Follow the scriptures lets read it all as it appreas in scripture..
Leviticus 23:KJV
7 In the first day ye shall have an holy convocation: ye shall do no servile work therein. 8 But ye shall offer an offering made by fire unto the LORD seven days: in the seventh day
is an holy convocation: ye shall do no servile work
therein.
So in the above scripture it says that during the 7days of the feast of unleavened bread a burn't offering must be made every day.. It says nothing about any first fruits It just says that a burn't offering must be made.. It does not even say that all people must make burn't offerings it may be that only the priest in the temple needed to make a burn't offering.. So we continue reading the scriptures and God declares that another offering must be made.. A different one:::
9 ¶ And the LORD spake unto Moses, saying, 10 Speak unto the children of Israel, and say unto them,
When ye be come into the land which I give unto you, and shall reap the harvest thereof, then ye shall
bring a sheaf of the firstfruits of your harvest unto the priest: 11 And he shall wave the sheaf before
the LORD, to be accepted for you: on the morrow after the sabbath the priest shall wave it.
And the Lord spake unto moses.. meaning that this is a seperate issue a seperate teaching not realted to the unleaved bread burnt offerings... This offering is the first fruitsand it has to be done after the Sabbath . that being a seventh day Sabbath.. So there is no problem if you understand that the two are different offerings.. The one that happens on every day of the feast of unleavened bread and the first fruit offering that only happens once on the first day after the weekly Sabbath