Isn't it part of the Sunnah to conclude the night with Witr instead of an even-numbered prayer? So why do the Malikis delay the post-Isha Sunnah prayer in Ramadan until after Tarawih and Witr?

In the Maliki school of thought, there is no Sunnah after Isha except for the Shaf' and Witr prayers. Therefore, Malikis perform Taraweeh after the remembrance of prayer directly during Ramadan. It is recommended to perform Nafl before and after Maghrib, Dhuhr, and Asr without a specific number of Rak'ahs. Nothing has been reported from Malik and his companions about Nafl before Isha except the general Hadith: \""Between every two calls to prayer is a prayer.\"" It is preferable that Witr be the last prayer of the night, and if someone prays Witr but then wants to perform more Nafl, there is no harm, but they do not repeat Witr. The best time for Qiyam (night prayer) is the last part of the night, unless someone fears they won't wake up, in which case they should perform Witr with the optional prayers at the beginning of the night.