It's because it is a partner flight. If you call CO, they can give you the UA reservation number, and you can log into the reservation on UA's website, where you'll be able to select seats.

I did the exact same thing on a LH award ticket purchased with CO miles last week. Once I selected seats on LH's website, the seats even showed in my account on CO's website (just the seat numbers, the seat map is still unavailable).