‘And as soon as it was day, the assembly of the elders of the people was gathered together, both chief priests and scribes, and they led him away into their council, saying,'
The Sanhedrin consisted of chief priests, Scribes, and lay elders/aristocrats of the people. These were now all gathered together, having hurriedly been assembled. Many would not have been pleased at having been dragged away from the festivities. But they had been made to recognise that the matter was important. So they were probably mainly concerned to get the matter over as soon as possible. Handing the matter over for Pilate to judge, especially as there appeared to be a capital charge involved, probably seemed a good idea.
We are clearly intended to read here ‘the elders of the people -- including the chief priests and Scribes', for Luke was well aware of the threefold nature of the Sanhedrin (Luke 9:22; Luke 20:1; Acts 4:5; Acts 4:23; Acts 6:12; etc.).