Why did Imam Bukhaari consider the hadith on fasting on the day of Arafat expiating 2 years worth of sins to be weak?

I've heard that imam Bukhaari did not consider the hadith in sahih Muslim on fasting on the day of Arafat expiating 2 years worth of sins to be authentic. https://www.abuaminaelias.com/fasting-on-day-of-arafat-or-not/ > ...He was asked about fasting on the day of 'Arafa (9th of Dhu'I-Hijja), whereupon he said: It expiates the sins of the preceding year and the coming year. ... (Sahih Muslim ) Why was that?