Si Mazhao is more ambitious and capable. Why did Sima Yi choose Sima Shi as his successor?

I think Sima Yi chose Sima Shi as his successor, mainly because the etiquette system at that time stipulated the rule of establishing the eldest son instead of the youngest son. Therefore, Sima Shi, the eldest son of Sima family, is the only candidate as heir. What's more, Sima Yi, as a person who knows etiquette, naturally won't violate etiquette and morality.

Moreover, Sima Shi has five daughters, but he has no son of his own. Is his only son, and Si Mazhao's second son to adopt in the past, so in the end, although Sima Yi handed the family to Sima Shi, but in the end, the power of the family will be transferred to Si Mazhao's hands. And Si Mazhao, as the younger brother of Sima Shi, can naturally live a little better than Sima Shi.

Moreover, Sima Shi, as the first person to take over Sima Yi's power, suppressed those who opposed Sima Jia's claim to the throne, and Si Mazhao, as the successor of Sima Shi, went to implement a loose policy for those who suppressed Sima Shi before, which naturally won the hearts of the people. In fact, these are just games of power, and they are all tricks used by Sima Jia to win people's hearts.

More importantly, Si Mazhao's personality is more open, and his ambition can be seen from his appearance. In contrast, his brother can keep his thoughts in his heart and is more forbearing. Therefore, Sima Shi is very suitable as the heir of Sima family.

Besides, the success of Gao Ping Ling's coup was also due to the death of 3,000 people in the Sima family. These dead people are all trained by Sima Shi, so in terms of etiquette and credit, Sima Shi is more suitable to be the heir of Sima Jia than Si Mazhao.