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.
First of all, you should know that some scholars – like Ibn Al-Qayyim – had chosen the view that the abrogation was not repeated in regard to the issue of Mut'ah marriage and that it was forbidden once on the day of the Conquest of Makkah. This means that it was initially permissible and then it was forbidden in the Conquest of Makkah.
If we presume that the abrogation was repeated, then there are some statements (from the scholars) about the reason why it was permitted again, in the sense that the Prophet gave permission for it after it had been forbidden. Ibn Hajar said in Fat-h Al-Baari: “We have already mentioned at the beginning of the book of marriage the Hadeeth by Ibn Mas’ood about the reason of permitting Mut'ah marriage, and that when they went on a military expedition, celibacy became so hard on them (i.e. they extremely desired their wives), so the Prophet gave them permission for temporary marriage. Hence, perhaps prohibiting Mut'ah marriage was repeated in each case after allowing it, and when the prohibition was made for the last time it was forbidden until the Day of Judgment, then there was no permission for it after that….”
Allaah Knows best.