Badspot-imposed limitation. Colorset system and gui was designed by him. Reason for this limitation im guessing is
1) cause of the gui size otherwise taking up most of the screen/needing a lot of customization and work to make it intuitive
2) less packet info needed to send to the client about the color of a brick (eg 1 digit instead of 4 floats) (helps when ghosting a ton of bricks)
3) using 5-6 datablocks per color adds up fast with lots of colors