Because of the IBJJF rules:
8.1.12
For Jiu-Jitsu No-Gi, athletes should abide by the following attire requirements:
• Men:
Board shorts primarily black with up to 30% of their rank color (belt), with no pockets or with pockets entirely stitched
shut, and reaching at least halfway down the thigh and at most to the knee. Attached metal or plastic of any kind that
may hurt the opponent is expressly forbidden on the board shorts.
Men are permitted the use of shorts or trunks of elastic fabric (clings to body), so long as they are worn under the
regulation shorts.
Shirt of elastic fabric (clings to body) at a length that covers the shorts’ waist band, of black or white color, and displaying
at least 10% of the color of the rank (belt) to which the athlete belongs. Shirts that are 100% the color of the rank (belt)
the athlete belongs to are also acceptable.
• Women:
Board shorts or lycra elastic pants primarily black with up to 30% of their rank color (belt), without pockets or with pockets
entirely stitched shut, and reaching at least halfway down the thigh and at most to the knee. Attached metal or plastic of
any kind that may hurt the opponent is expressly forbidden on the board shorts.
Shirt of elastic fabric (clings to body) at a length that covers the shorts’ waist band, of black or white color, and displaying
at least 10% of the color of the rank (belt) to which the athlete belongs. Shirts that are 100% the color of the rank (belt)
the athlete belongs to are also acceptable.
http://www.ibjjf.org/docs/rulesibjjf1stedition.pdf