I don't think the severity is the explanation. Australian incidents reported to ICAO included gear collapse with no injuries. Based on this, a ditching with hull loss and significant injuries should meet the reporting threshold.
There may be some gamesmanship regarding class of operation.
However the intent of the ICAO seems pretty clear.
From what I have heard, a very senior ICAO person is surprised to be unaware of Norfolk.
Even more oddly, there are some incidents that were reported to ICAO for which there is no ATSB report (or at least, the ATSB report can not be found on the ATSB site). For example, the following PelAir / Rex incidents.
25/11/2004 VH-EEX Metro. Port gear collapsed on landing at Rockhampton ATSB reference 200404619 (I eventually found the ATSB record so ATSB know of incident, but could not locate the actual report).
3/4/2007 VH-KDO Metro Can’t find anything at all about this.
Probably not major incidents but odd that they have been reported to ICAO and we can't find any ATSB report.