Why did Bukhari and Muslim leave out some sound hadiths despite knowing their authenticity, and did those hadiths reach them but were not included in their Sahihs?

Al-Bukhari and Muslim did not intend to include all authentic hadiths in their Sahih (definition) collections. This is evidenced by Al-Bukhari's naming his book "Al-Mukhtasar" (The Abridged), and Muslim's clear statement that not every authentic hadith (definition) he possessed was included in his Sahih. Furthermore, Al-Hafiz Ibn Hajar and Al-Sakhawi affirmed that Al-Bukhari and Muslim explicitly stated their intention not to be exhaustive, and that the reason for omitting some authentic hadiths was the fear of excessive length. A hadith might also be omitted due to a defect in it, or forgetfulness, or because what they did mention sufficiently served its purpose.