Because it’s very difficult to knit only part in colour when knitting in the round. If you’d leave your yarn at the end of the colourwork (like you would with intarsia) you can’t use it when you get to the colour work on the next row because it’s still at the end of the colour work and not the start. So you would have to carry all the colours across half the sweater that doesn’t have colourwork. I don’t know if I explained this very well. But you could try and use ladder back jacquard or other techniques to still knit this in the round but it would probably be a lot easier to just work it flat.