Why is it necessary for Muslim men to keep their ankles exposed or naked.?
Folding up the legware is related to a particular form of dress called the Tehmad (loin cloth). In the times of the Prophet (sws), some people would wear it in a manner that a portion of it would drag behind them. Since this is an expression of arrogance and haughtiness, the Prophet (sws) forbade wearing it in this manner. This directive to fold up legware is therefore not related to forms of dress which are not worn in this manner or this purpose, for example pants, trousers or shalwars.
Also, even in case a person is wearing a Tehmad and his intentions are not to express such arrogance, exceptions can be given -- as is narrated in the following Hadith:
The Prophet is reported to have said: A person who makes his cloth to dangle after him will not be looked at by the Almighty on the Day of Judgement. At this Abu Bakr said: A portion of my cloth drags on the ground unless I am very careful about it. At this the Prophet replied: You do not do it out of arrogance [so your matter is different]. (Bukhārī, Kitābu’l-Libās)
It is imperative that a the basis as well as the nature of a directive mentioned in a Hadith be analyzed before following it. Arrogance and haughtiness are condemned by the Qur’ān. So all their manifestations and symbols have been prohibited by the Prophet (sws).