The simplest explanation is Matthew did not call it the Sabbath because it was not the Sabbath.
Literally, the seventh day weekly Sabbath is described by two words. It is a "sabbath of rest:"
“Six days shall work be done, but on the seventh day is a Sabbath of solemn rest (שַׁבַּ֤ת שַׁבָּתוֹן֙), a holy convocation. You shall do no work. It is a Sabbath to the Lord in all your dwelling places. (Leviticus 23:3) [ESV]
In addition to the 52 weekly sabbaths of rest, the entire 24-hours of Atonement is described as a day of Sabbath rest:
It shall be to you a Sabbath of solemn rest (שַׁבַּ֤ת שַׁבָּתוֹן֙), and you shall afflict yourselves. On the ninth day of the month beginning at evening, from evening to evening shall you keep your Sabbath (שַׁבַּתְּכֶֽם).” (Leviticus 23:32)
There are other days on the calendar on which work is prohibited. However, those days of rest are not specifically called a "Sabbath of rest." In particular, the day after the day of preparation is the first day of the Feast of Unleavened Bread and it is a day on which work is prohibited:
And on the fifteenth day of the same month is the Feast of Unleavened Bread to the Lord; for seven days you shall eat unleavened bread. On the first day you shall have a holy convocation; you shall not do any ordinary work. (Leviticus 23:6-7)
Neither of the words used to described the Sabbath are used to describe the first day of Unleavened Bread. One of the distinctions between the two is what is commanded:
Sabbath command: rest
Unleavened Bread command: do no ordinary work
Avoiding "ordinary work" does not necessarily mean a person will observe a day of solemn rest; while observing a day of solemn rest means a person will avoid all types of work, including "ordinary work." Regardless of actual practices, the different language in Leviticus is such to allow some "work" on the fist day of Unleavened Bread which is prohibited on the weekly Sabbath and the Day of Atonement.
Matthew has preserved this distinction between the first day of Unleavened Bread and the weekly Sabbath:
The next day, that is, after the day of Preparation, the chief priests and the Pharisees gathered before Pilate...Now after the Sabbath... (27:62, 28:1)