All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Indeed there are many Prophetic narrations which state that it is forbidden to wear garments below the ankle; some of these narrations clarify the wisdom for such a prohibition. Among these reasons are the following:
1- Trailing the lower garment below the ankle is considered as conceit. The Prophet said: "Beware of trailing the lower garment, for it is conceit." [Ahmad] It appears from this narration that wearing the garment below the ankle is considered as conceit even if the person who is wearing his garment this way did not intend it. Ibn Hajar said: 'In conclusion, wearing a garment below the ankle implies trailing it below the ankle-bone, which implies conceit even if the person who wears his garment this way did not intend it [conceit].'
2- By trailing the lower garment below the ankle, it is most likely assumed that this will cause it to be stained with impurity and other dirty things on the floor. 'Umar pointed to this when he saw a young man trailing his garment on the floor, as he said: "Bring that young man to me, and he told him: "O son! Raise your garment as by doing so, you will keep your garment clean and you achieve more piety [you become more Allaah-fearing]." [Al-Bukhari]
3- By trailing the lower garment below the ankle, one may resemble women as the Prophet ordered women to trail their dress on the floor by one cubit's length [more than men], as per the narration narrated by Um Salamah and reported by Ahmad. Therefore, if a man lowers his garment below the ankle he is in a way resembling women, and the Prophet said: "Allaah cursed men who imitate women." [Al-Bukhari]
So, a Muslim is required to stick to Allaah's Commandments and the orders of the Prophet whether or not he knows the wisdom behind such orders. Allaah Says (which means): {It is not for a believing man or a believing woman, when Allaah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allaah and His Messenger has certainly strayed into clear error.}[Quran 33:36]. Therefore, if Allaah or His Prophet prohibit us from something, it is an obligation to obey whether or not we know the wisdom behind it, and this [submission] is the essence of Islam. However, if we obey only when we know the wisdom and we reject and refuse to abide by the order if we do not know the wisdom, then this is contrary to what Allaah and His Prophet have ordered us, and this is contrary to submitting oneself to the Divine orders.
Allaah Knows best.