Evolution and geographic spread of Cyanea, the largest genus of native Hawaiian plants

Ah - it turns out that ALL species of Cyanea are restricted to the Hawaiian Islands. Presumably, other islands in the Pacific are simply too far away to colonize. What is truly surprising is that 93% of Cyanea species are restricted to individual islands, such as Kaua`i, O`ahu, Maui, and the Big Island of Hawai`i. This despite the fact that these islands are quite close to each other, and despite Cyanea having fleshy fruits eaten by birds that should disperse the seeds far and wide. But most Cyanea species grow in the understory of tropical forests, and most understory birds are notoriously sedentary. As a consequence, we believe that – as with a number of understory genera of flowering plants – populations of individual species can undergo genetic differentiation at small spatial scales, leading ultimately to high rates of speciation and narrowly distributed species – generating a lot of biological diversity, but leaving many of the resulting species highly prone to extinction as well.

I'm curious! What is your hypothesis as to why over 90% of the Cyanea genus is restricted to Hawaii? Why aren't they able to thrive on other islands with the same type of ecosystem?