My understanding is that they simply haven't completed negotiations with HAL yet. In fact, under "program news" on the MR website there is a link that brags about new 2006 HAL cruise options, but when you click on it, it takes you to the T&C page that shows only Carnival cruises available.

I've taken two HAL cruises with MR points (and paid to upgrade to wonderful cabins) and think it is an outstanding value for someone like me who would rather earn points staying in a hotel than burning them for the same nights.