Bella is still very much in demand, and Hailey, who Anna gave a cover to a couple of years ago, probably has more major covers than anyone else so far in 2021, so she's seemingly in demand as well. I'd agree with you if this was Anna's first time featuring any of them, but it's not so I don't get that particular critique in regards to these two.She's not late to feature them, relative to the rest of the industry.
As for Kaia, I'm pretty sure she was not originally intended to be the cover subject and the decision to turn her ed into a cover feature was likely made because Bella has a cover coming in a few months (and her look in the ed isn't cover-friendly) and Hailey was just on US HB, US Elle, Vogue Paris, etc. etc. etc. so it would seem late-to-the-party of US Vogue to give her a cover, even though the shot of her in the checkered Versace top would have made a much better cover than Kaia's imo.