I'm wondering why Marcus Aurelius, who could be considered as one of the most wise emperors of Roman Empire, did not restore Roman Republic constitutions? I read somewhere that the functionality of Roman Republic were quite lost at that time but I think it's not convincing cause some imperial era historians such as "Suetonius" were quite familiar with Roman Republic history and its constitutions. I appreciate any suggestion or recommendation.
That's a common misconception about this emperor. Julius Caesar made a huge mistake by working his way towards becoming an emperor. His successor Octavianus Augustus didn't make that mistake. He became an absolute ruling emperor but under the guise of a republic. This guise was continued by all emperors. But gradually they became emperor in their own right.
Marcus Aurelius ruled from 161-180 AD. That is almost 2 centuries after the first emperor. By that time, not many seriously believed in a republic. That was something from the dim past. Marcus groomed both Commodus and Marcus Annius Verus for the purple.
There was never doubt they wouldn't succeed him or the republic would be restored. I think (but am not sure) the republican preference of Marcus is very much a modern idea.
I remember researching Claudius to absolve Nero of the guilt of being exceptionally nasty as an emperor. The distinct impression I got was that Augustus was exceptionally clever and devised a very clever system, provided an exceptionally clever emperor was doing the job. Early Roman Empire could work, but had very little trust built into it, no significant staff-level bureaucracy and no tamper-proof succession mechanism.
Augustus was smart enough to do the job. Caesar would have been. Few of the later emperors could sustain those demands.
This does not mean it was an obvious failure compared to Republican times. Punic War One is full of dysfunctional results because of mandated daily leadership alternance at the Legion-plus level. The circumstances of the Crassus-Caesar-Pompeii Triumvirate and Marius are pre-Empire. The "Roman Republic" sounds good but by any modern metric it was an insanely violent, non-representative, and corrupt system, where politicians could serve for no pay, not just because they were rich enough to buy elections, but because they could expect to become richer still by patronage and graft. The Empire did not change that much for the worse in practical terms, except for the Legions becoming acknowledged kingmakers.
The notion of wise and benevolent dictatorship has always been attractive and once a dictator sits the throne he is always both and will choose the best heir for his nation.