Why were fasting and Hajj not mentioned in the Messenger's (PBUH) exhortation to Mu'adh ibn Jabal when he sent him to Yemen, and he only sufficed with the two Shahadas, prayer, and zakat?

The hadith of Mu'adh was narrated by Al-Bukhari and others, and it does not mention fasting or Hajj. This is because the Sharia's emphasis on prayer and zakat is greater, which is why they are repeated in the Quran. They never fall away from the مكلف (accountable person), unlike fasting, which can be waived for a chronically ill person whose recovery is not hoped for, or for a very old person, both of whom must pay a fidyah (expiation). Similarly, Hajj can be waived for a disabled person who cannot perform Hajj due to chronic illness or incapacitation.