Because we want a stop gap for a few years while English matures, and Casboult would cost a late pick vs De Koning a 21 year old developing ruckman.
If we are considering Ratugolea and Mason Cox, I don't think Casboult is much different to them. Ratugolea is younger and more athletic, and is probably showing similar signs to Casboult early in his career (can take a mark, but then cover your eyes). Mason Cox has played a handful of good games in his career, but admittedly they have all come in finals.