Why are there only indirect references to Imamat in the Quran instead of having a direct command, such as a verse that could have asked the Muslims to follow the 12 successors of Rasoolullah?
There is very clear evidence in Quran to follow The Twelve Imams after the Prophet Muhammad (SAWA). Allah (SWT) says: Obey whatever the Prophet tells you, and avoid whatever he orders you to avoid. (Sura 59, Verse 7). This order includes everything. That is why we don't find in Quran the mention of number of Rak'ats of our obligatory prayers (two for morning, 4 for Dhuhr, 4 for ASR, 3 four Maghrib and 4 for Esha) , but we took it from the Prophet (SAWA).
The Prophet (SAWA) has clearly stated that his successors will be twelve in Authentic Hadeeths narrated in Sunni and Shia books.
Wassalam.