This and obviously the Chromium content.
There are many high-hardness high wear-resistant Chromium-Vanadium steel nowadays, and many are designed specifically to have a relatively high corrosion resistance (20CV, S90V, S110V, S125V, etc ...), but as it is well known they have a strict HT which leads to a high HRC but certainly not to the maximum attainable.
This is not the case of Zdp189.
Zdp189 is created to have a superhigh hardness (i've seen it at 69 HRc), a S90V-class wear resistance and still maintaining an ease of sharpening way higher than S90V.
How it achieve this wear resistance? By Chromium carbides, that is the only motive why Chromium is here.
Mr. Kitano, the developer never cared of corrosion resistance, he wanted ease of sharpening and superhigh hardness/wear resistance.
This is the same motive why D2 has 12% Cr.