When Paul says "I don't allow women to teach" why is this read as an instruction?

It's an interesting argument from Paul. I don't let women teach because Eve was deceived. Huh? Aren't there numerous examples in the Bible of men being deceived? Are all women deceived?

It would seem something else is going on that troubles Paul at the time. After all, in the next letter to Timothy, Paul praises Timothy's grandmother and mother for instilling unfeigned faith in him.

When I call to remembrance the unfeigned faith that is in thee, which dwelt first in thy grandmother Lois, and thy mother Eunice; and I am persuaded that in thee also. 2 Tim 1:5

Why was Eve deceived? At the time, there was no written Bible. The snake asked did God really say? Paul knows there are deceitful men, wolves in sheep's clothing.

But evil men and seducers shall wax worse and worse, deceiving, and being deceived. 2 Time 3:13

What is the solution that Paul is waiting on?

And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus. 2 Tim 3:15

Timothy's women teachers knew the Old Testament and some of the New, but the New was not yet codified, not yet fully written.

All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 2 Tim 3:16

All scripture had not been written. All scripture is profitable for doctrine, reproof, correction, for instruction. Until then, it was important to restrict the sex of the teachers. In Christ, however, there is no male or female. From an instruction point of view, it simply awaited the canonization of Scripture.

So, why do some still believe women should be silent? Perhaps they teach Tradition, the unwritten did God say?

Paul says "I" simply because this was personal letter (1:1-2), and he is writing to personally instruct someone (Timothy) how to conduct themselves in the church (1:3ff), and to command what he writes to others (1:3, 4:11).

Do you think his "urge" in 2:1-2 was time-bound or only meant for the recipient? ("I urge that requests, prayers, intercessions, and thanks be offered on behalf of all people, 2 even for kings and all who are in authority, that we may lead a peaceful and quiet life in all godliness and dignity")

Of course not! The very next verse says, "Such prayer for all is good and welcomed before God our Savior".

In other words, this epistle is authoritative, Apostolic instruction to the church.

Delivered in singular form to Timothy, but intended to be spread far and wide.

Paul says elsewhere, Be ye followers of me, as I am of Christ. That is an imperative, a command, and includes all that he spoke ; and all of his, behaviour as an example.

Nothing that Paul said or did can be excluded from his command :

Be ye followers of me, even as I also am of Christ. [1 Corinthians 11:1 KJV]

My church people (a branch heavily influenced by the Anabaptist movement) teach that this example is teaching for the church. Building a bit on Nigel J's answer (who can freely contradict me, if he would like, in the comments):

A bit of a side-note: This passage does not specifically mention church leadership; it is more general. As you can see, it tells men "everywhere" to pray, lifting up holy hands, and the whole tone of this part of the letter is rather broad than restricted.

But the apostle taught it, and we take it as teaching for the Church, since he mentions "women who fear God", and we suppose that the men who are supposed to pray everywhere also part of the church by being made disciples and being born again. This view that apostolic example is an example for the church in general is in line with the "Regulative Principle", which teaches roughly that if Christ and the apostles as a group can be shown to have taught and practiced something,the church should obey and follow their example (especially where it touches public worship); otherwise (if Christ and the apostles did not teach and practice some doctrine), it is not for the church to innovate.

Concerning the specific doctrine of man-only church leadership, it is given more particularly (or reinforced, if you take the I Tim. passage as foundational to the teaching) in the letters to the Corinthians:

Women should remain silent in the churches. They are not allowed to speak, but must be in submission, as the law says. If they want to inquire about something, they should ask their own husbands at home; for it is disgraceful for a woman to speak in the church. (I Cor 14:34-35)

For the record, we do not teach that "women should be subservient to men, should play no real part in society and [a]re incapable of intellectual discourse to the same extent as men", as DJ's answer points out, at least in theory (I'll just say that we have varying degrees of success in carrying out the practical equality of value that is clearly taught by the apostles elsewhere). Indeed, many of the women among us are intellectually superior to the men, and better at discourse, and more socially able. In stead of "subservient", we teach that the women "should be quiet and learn in submission" (as Paul taught in the passage that you quote), especially in the church.

There are really two reasons why Christians have taken this passage as a command rather than a statement of Paul's current practice.

First Paul enjoys a very special place in the history of the early church, having written a huge amount of the New Testament (as believed over most of the last 2000 years) and his founding of so many early churches. Many Christians take the authority of his commands as second only to Jesus himself (in the case of Dispensationalists, even more authoritative than Jesus). It is unsurprising that if Paul writes "this is how I do something" then many Christians take it as "this is how it should be done".

Secondly Paul's practice in this area has agreed with the expectations of the culture over almost all of the last 2000 years. In almost all of that time it has been almost universally believed that women should be subservient to men, should play no real part in society and were incapable of intellectual discourse to the same extent as men. There are exceptions to this norm, but it has been the norm for 2000 years, at least in the parts of the world where Christianity flourished.

The view that women should be permitted to rule over men would be considered radical and contrary to natural order for most of this period. It is unsurprising that any passage seeming to gainsay this view would receive wide acceptance.

Similarly, an author can write, my god, my gospel etc. it doesn't mean it is a personal unique opinion. On Romans 2:16, "according to my gospel", Utley comments:

"according to my gospel" In context this referred to Paul's preaching of the revelation of Jesus Christ. The pronoun "my" reflected Paul's understanding of the stewardship of the gospel that had been entrusted to him (cf. Rom. 16:25; 1 Cor. 15:1; Gal. 1:11; 1 Tim. 1:11; 2 Tim. 2:8). It was not uniquely his, but as the Apostle to the Gentiles he felt an awesome sense of responsibility for spreading the truth about Jesus in the Greco-Roman world.

Paul does forbid women from speaking and teaching in the assembly and to have an authoritative role in the ministries that includes elders, pastors position 1Tim 2:11-12; 1Cor 14:33-38, it was his rule as he says, "in all the churches", no commandment was unique to one church and time. It says, I write to you, that they are the commandment of the Lord. In Timothy, he adds the authority of the Genesis story of Adam Eve, by reminding them of the commands and tradition of the law, none of which are his personal opinion, but law.