Why don't we have a single descendant of Hassan among the 12 imams?

To the Shi’a, the Ahlul-Bayt of the Prophet (S) consist of the following individuals ONLY: • Fatimah al-Zahra (sa), • Imam ‘Ali (as), • Imam al-Hasan (as), • Imam al-Husayn (as), • Nine descendants of Imam al-Husayn (as) Reference : https://www.al-islam.org/shiite-encyclopedia-ahlul-bayt-dilp-team/who-are-ahlul-bayt-part-1 Question: why don't we have a single descendant of Hassan among the 12 imams