It's because of fear of getting in trouble with an anti gun state for violation of their laws. Most vendors don't have a lawyer look all the state and local laws from all 50 states. It's just too cumbersome and changes too often. instead someone in the company will read the HRS online and say "let's play it safe". Another reason for this problem is how stupidly the HRS pistol mag cap ban is written. it's vague and open to interpretation. The biggest is the usual AR or AK mag thing because is it a pistol mag, or a rifle mag? It can fit both. The bottom feeders will say it's banned under HRS because it's capable of being used in a firearm designated as a pistol. We argue it's a rifle mag because it fits in our AR rifles and besides,Detachable mag AR pistols are already outlawed for purchase and importation since the mid 90s. People have been arrested and charged with "high" cap pistol mag violations for having a standard AR mag. Of course this person was shooting up a bank at the time so the prosecutor just added on more charges on top of robbery. Has anyone ever been convicted of Hawaii's pistol mag law for having and AR or AK mag? not that I know of.
in short, many vendors don't want to Guinea pig the law since any legal victory also comes with potentially business bankrupting legal bills