Why does the Council of Vocations assign Equality the job of street sweeper?
Equality 7-2521 gets the job of "street sweeper" for no other reason than that is what is selected for him by the Council of Vocations. When he went to find out his fate, he was hoping to be named a scholar. This wish was a sin in the society. That is why, when Equality 7-2521 learns of his fate, he says he is happy:
We felt the cords of our neck grow tight as our head rose higher to look upon the faces of the Council, and we were happy. We knew we had been guilty, but now we had a way to atone for it.
The society has trained him to be happy about whatever fate he receives. He must portray a sense of happiness outwardly, but he also is a little happy on the inside because of his conditioning. If the society allowed its citizens to be involved in the selection of their vocations, it would fail miserably.
Moreover, Equality is taller than the others, physically symbolizing difference which is not allowed in the dystopian society.
I think the intent of the council is more to relegate him to a position of non-threatening uselessness rather than any direct attempt to break his pride / will. He is only noticed in a negative way when he brings the light box to the World Council and causes such a violent reaction. In essence, before he discovers the tunnels, he is completely incapable of causing a problem because he is surrounded by other controlled people, who, like all members of a dystopian world, are fully charged to tell the authorities if he made illegal statements which betrayed his thought life into real words.
Throughout his life, Equality 7-2521 has caused trouble by being smarter than the others. He secretly longs to become part of the Council of Scholars. The rulers of his society fear that his intellect will lead to things that will bring about technological advances that they are unprepared to deal with, so he is assigned a menial job in the hopes that he will dutifully accept his fate and live a life that does not threaten the society the leaders have worked so hard to maintain.
He shows signs of intelligence and motivation - things which would be rewarded in a capitalist society, but which are unacceptable when trying to control people and put them in their place. He is assigned as a street sweeper so he doesn't think.
Also, in addition to his intellect, Equality 7-2521 possesses detectable pride, and by assigning him a menial position, the Council hopes to break his pride, thus making him more compliant to their will.
Equality 7-2521 shows signs of intelligence and skepticism since his youth. Being superior over his other brothers is considered a sin in their society. The society in Anthem does not want to promote such egotistical characters. As a result, to crush his pride, the council selects him to be a street sweeper so that he would not be able to do much and would have a 'lower' position in society.