Vega is supposed to be the reference for the brightness of stars with a visual magnitude ~ 0.
However, the Bright Star Catalogue lists four sky objects that are brighter than Vega (the lower the magnitude the brighter the star). Does anyone know why, say, Sirius is not the benchmark rather than Vega?

Vega
Constellation Lyra
Star Name 3alp Lyr
Observed Visual Magnitude (V) 0.03

Rigel Kent
Constellation Centaurus
Star Name alp1Cen
Observed Visual Magnitude (V) -0.01

Arcturus
Constellation Boötes
Star Name 16alp Boo
Observed Visual Magnitude (V) 0.16 (given as -0.04 in BSC)

Canopus
Constellation Carina
Star Name alp Car
Observed Visual Magnitude (V) -0.63

Sirius
Constellation Canis Major
Star Name 9alp CMa
Observed Visual Magnitude (V) -1.44