[Machine] I do not know why the Messenger of Allah, ﷺ , prohibited it. Perhaps it was because it was burdensome for the people, so he disliked their burdens being taken away or prohibited them from eating the flesh of domesticated donkeys on the day of Khaybar.
لَا أَدْرِي أَنَهَى عَنْهُ رَسُولُ اللهِ ﷺ مِنْ أَجْلِ أَنَّهُ كَانَ حَمُولَةَ النَّاسِ فَكَرِهَ أَنْ تَذْهَبَ حَمُولَتُهُمْ أَوْ حَرَّمَهُ فِي يَوْمِ خَيْبَرَ لَحْمَ الْحُمُرِ الْأَهْلِيَّةِ
I do not know whether the Prophet ﷺ forbade the eating of donkey-meat (temporarily) because they were the beasts of burden for the people, and he disliked that their means of transportation should be lost, or he forbade it on the day of Khaibar permanently.
لاَ أَدْرِي أَنَهَى عَنْهُ رَسُولُ اللَّهِ ﷺ مِنْ أَجْلِ أَنَّهُ كَانَ حَمُولَةَ النَّاسِ فَكَرِهَ أَنْ تَذْهَبَ حَمُولَتُهُمْ أَوْ حَرَّمَهُ فِي يَوْمِ خَيْبَرَ لَحْمَ الْحُمُرِ الأَهْلِيَّةِ
I do not know whether Messenger of Allah ﷺ prohibited (the eating of the domestic ass) due to the fact that they were the beasts of burden for the people, so he (the Holy Prophet) did not like their beasts of burden to be destroyed (as a matter of expediency), or he prohibited the use of the flesh of domestic asses (not as an expediency but as a law of the Shari'ah) on the Day of Khaibar.
لاَ أَدْرِي إِنَّمَا نَهَى عَنْهُ رَسُولُ اللَّهِ ﷺ مِنْ أَجْلِ أَنَّهُ كَانَ حَمُولَةَ النَّاسِ فَكَرِهَ أَنْ تَذْهَبَ حَمُولَتُهُمْ أَوْ حَرَّمَهُ فِي يَوْمِ خَيْبَرَ لُحُومَ الْحُمُرِ الأَهْلِيَّةِ