Why are Eastern Rite married men allowed to be ordained priests?

Because priestly celibacy is a discipline, not a doctrine, and so there is room for diversity on the issue according to the customs of the respective rites. If celibacy were a doctrine, all rites would have to conform to the judgment of the Holy See on the matter because doctrines are true for everybody. But celibacy is a discipline (a practice that is legislated by proper ecclesial authority) that has been deemed to be spiritually beneficial. In the Latin rite, this spiritual discipline ordinarily is required of all men who seek priestly ordination. In the Eastern rites, it is practiced by the monks and by some secular priests, but it is not required of all men who seek ordination. Out of respect for the longstanding customs of the Eastern-rite churches, the Vatican allows the Eastern churches in communion with the Holy See to maintain their own properly constituted discipline on this issue.