BCAAs, particularly leucine, are important for optimizing the signal to build muscle, but you need the rest of the amino acids to actually build muscle. BCAAs make up 16-25% of the protein sources we typically eat (fish, meat, whey powder), so there is no need to supplement them. Supplementing with BCAA alone would be like getting the construction workers on a building site fired up to work for the day, then not giving them any materials to build with.