There are a few reasons: Kava high in DHM makes you nauseous, and is not entirely pleasant to the vast majority of kava drinkers. This fact has guided the creation of most cultivars to be high in kavain and DHK. Even most cultivars that people on here would say are high in DHM, such as our Boroguru at BKH, still have much higher concentrations of other kavalactones, but slightly higher amounts of DHM.
Furthermore, most kava comes from Vanuatu which in theory only allows the exportation of noble kava strains which are always going to be daily drinking kavas (relatively low DHM, higher kavain and DHK). The other big exporters: Fiji, Tonga, and Hawaii simply grow very little strains with high DHM because people don't like to get sick and feel kava'd out for more than a couple hours.
All that being said, the PNG supplier I found who has sent me a 10 kg. sample that I'm currently waiting on has described the kava he sells as a strain that provides a "halo" effect the next day. The farmers call it "Koniak". These two facts lead me to believe it is a tudei strain, higher in DHM than those available from elsewhere in the Pacific. If all goes well and the kava bar customers like the samples of this stuff we'll be adding it to the lineup online shortly.