Quite simply Cost! Colors are just as good and they cure the same, I've mixed custom colors with them and glitter. I have powder from Columbia, Roseys, Prismatic, Eastman and Orbit.
Even with quantity you can trade others for other colors you don't have.
Columbia you can get a 1/2 lb. on some colors for $7-$9. A bunch of online tackle stores charge anywhere from $6 and up for 2 ozs.
I've also used Jacquards Pearl Ex mica powders mixed with clear powder paint, and it works great.