Because God said it was. Although He doesn't clearly explain why, there are clues in Scripture. The reference to Adam and Eve in 1 Timothy is most likely refuting the false teaching that Eve was created first and had secret knowledge. Paul refutes it by saying that Eve was deceived (rather than knowledgeable).
1 Tim 2:
11A woman
a should learn in quietness and full submission.
12I do not permit a woman to teach or to assume authority over a man;
b she must be quiet.
13For Adam was formed first, then Eve.
14And Adam was not the one deceived; it was the woman who was deceived and became a sinner.
15But women
c will be saved through childbearing—if they continue in faith, love and holiness with propriety.
Paul isn't talking about order of creation but about authority/dominion. And i doubt there's any group that claimed Eve was created first when old testament scriptures says the opposite. Paul says even though Eve was deceived and sinned first, the sin was put on the account of her husband because of the dominion/authority.
If we see it this way, then even us who have sinned would believe the atonement work by Christ because if we submit to Him, our sins are put on His account and are all paid. If we don't see it that way, we can't recognize Christ as having authority/dominion over us as our husband. This is the essence of salvation that was set long ago in the garden of Eden.
It is about authority and nothing else. Through authority sin affects everything and everyone in the world and through authority authority sin is defeated.
Paul is always talking about the spiritual authority and we must be aware of it and not represent his context. This is a clear correlation:
Eph 5:
22Wives, submit yourselves to your own husbands as you do to the Lord.
23For the husband is the head of the wife as Christ is the head of the church, his body, of which he is the Savior.
24Now as the church submits to Christ, so also wives should submit to their husbands in everything.
25Husbands, love your wives, just as Christ loved the church and gave himself up for her
26to make her holy, cleansing
b her by the washing with water through the word,
27and to present her to himself as a radiant church, without stain or wrinkle or any other blemish, but holy and blameless.
28In this same way, husbands ought to love their wives as their own bodies. He who loves his wife loves himself.
29After all, no one ever hated their own body, but they feed and care for their body, just as Christ does the church—
30for we are members of his body.
31“For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh.”
c 32This is a profound mystery—but I am talking about Christ and the church.
33However, each one of you also must love his wife as he loves himself, and the wife must respect her husband.
