The First Code of the Knights of Camelot states that only those of noble blood can become knights (Lancelot). This was because Uther Pendragon only trusted the higher class, who had sworn allegiance to him and had supported his rise to the Throne of Camelot, and their sons became knights to honour this trust. Although The First Code was unfair and not popular, it still was in use when the series began and was considered to be a rule which unified the knights.
It was because of the decree of The First Code that Lancelot was not allowed to join the knights, despite Arthur having vouched for his noble nature (Lancelot).
Arthur later abandoned this code, by knighting Elyan, Lancelot, and Percival, who were not of noble birth, and Gwaine, who Arthur did not know was of noble blood (The Coming of Arthur).