“Married only once” doesn’t necessarily mean they have to have married at all. There is nothing to intrinsically prevent a married man being ordained, but the Church has grown considerably since that letter was written. The role of a priest or bishop involves a lot more. Not to mention that at the time, most people would have been converts, and most converts would have been married, so to prohibit married men from being ordained simply wouldn’t have worked.
But that passage doesn’t it make clear that it is ok for “Bishops” to marry?
It makes it clear that there’s no moral reason why they can’t, yes. But that doesn’t mean that the Church can’t impose laws for the good of the Church. There’s nothing intrinsically wrong with eating meat on Friday, but the Church thought it fitting to set aside Friday as a mandatory day for the commemeration of Jesus’ crucifixion and death. And with celibacy, the Church has realized that celibacy “for the sake of the kingdom,” as Jesus and Paul recommend, is of great value for the Church.
Theoretically, in the future, married men could become priests and bishops outside of the Eastern Churches or extraordinary circumstances. There are good reasons for maintaining the Church’s current discipline (i.e. policy), however, and I don’t think it will change anytime soon, unless there is a big push for it from the clergy.