When we proposed it, T2Diabetes wasn't in the mix. Even insulin dependent Diabetes wasn't in the mix. ESRD, a collection of enzyme-related orphan diseases, some terminal cancers with high end-of-life costs, hemophilia, etc. were in there.
Diabetes, regardless of type, is generally within about 3 sd of the mean for the annual claim probability distribution - well within the range of expected variance. Hemophilia, when an event occurs, ends up being up around 6 to 10 sd. The things we wanted in the HRP are all things that are waaaaaaay out on the tail of the distribution, but that have a material effect on the outcome of the pool as a whole. The whole point of it is that it's for a very small set of people with very extraordinary medical needs and costs.
It's like the opposite of a lottery. Right now, if you win the lottery, you get some massive upside potential and nobody is unhappy. Imagine if it were the other way around - imagine if the lottery would give you a multi-million dollar debt. Nobody wants that. It makes more sense to take that very rare occurrence of OMG skyrocketed costs, and share them out over a bigger population.