The Rabbenu Bechaya at the end of Parashas Ki Siso explains the expression חסורי מחסרא והכי קתני Rebbi only wrote down what was absolutely necessary and not what was self understood from the mishna. When the generations became weaker in knowledge, more had to be written down. This is how the gemora came into being. So what was left out of the mishna, was left out on purpose.
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