There were 300 of them. They were split between three points around the intersection where the Police station is located.
Lets say a bloke has been identified as needing arresting in the second rank of people A PC goes for him, so he moves back. PC goes after him.
Seeing this 3 or 4 other cops go after him.
PC 1 catches the man who starts to fight. The crowd around him start to assault the cop who has got him, the other cops arrive and are immediately fighting with the crowd as well.
You then have 4/5 cops against the mob.
Do you order the line forward to rescue and risk the majority of the mob being able to bypass the line and reach the Police station and attack the other cordons from behind?
Or do stay put, keep your troops relatively safe and protect the Police station?
Say an arrest was made without the above scenario. How many Police do you think would be needed to remove that prisoner from the scene? He isn't going to come quietly so you'd need an arresting officer, a witness and at least two other cops as escort. You'd need a vehicle with a driver to transport the prisoner and probably a group of 20 officers to prevent a rescue. How does that leave your cordon?
Do not think that there wasn't an officer there who didn't want to make arrests, or at least clear the street, but when faced with a gang like that who outnumber you by at least 3 to 1 you have to take a broader view.