The wisdom behind the late prescription of fasting the ninth of Muharram
Why did the Prophet not order the Companions to fast the ninth of Muharram except a year before his death, despite the fact that the Jews left Madeenah a long time before this?
All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.
The answer of this question was mentioned by Al-Haafith Ibn Hajar as he said,
Ahmad narrated this Marfoo‘ Hadeeth (i.e., directly attributed to the Prophet ) on the authority of Ibn ‘Abbaas: 'Fast the tenth of Muharram and contradict the Jews by fasting one day before it or after it.' This was at the end as the Prophet used to do the same as the People of the Book in matters where he was not ordered to do otherwise, especially when this would be contradicting the idolaters. However, when Makkah was conquered and Islam became widespread, the Prophet liked differ from the People of the Book, as what was narrated in As-Saheeh. The Prophet did as the People of the Book did initially, then later said: 'We are worthier of Moosa than them.' Then he liked to be different from them and ordered us to add one day before it or after it.
Thus, the Prophet intended to fast the ninth of Muharram to be different from the Jews when Islam was widespread and its followers became many, but before this he used to do the same as the People of the Book in matters that do not include order or forbiddance from Allah The Almighty.
Allah Knows best.