https://www.nzfootball.co.nz/newsarticle/88261?newsfeedId=760052
The fact that Des was quietly moved on from the U20 job suggests that this is NZF-driven, rather than Des-driven (not to mention the rumour mill on this, which is obviously difficult to verify here). The U23 situation is a continuation of that.
Before anything else, the U-20 job (OFC U-19 Championship in late July) was (pre-Covid-19) incompatible with the U-23 job (Tokyo Olympics in late July).