There are lots of different variables that influence adherence, including but not limited to:

- expected or actual efficacy

- regimen complexity

- knowledge, commitment, and ability

- doctor-patient relationship

My understanding is that with HIV a lot of money has been spent on actually implementing the adherence research in order to reduce the spread of the disease, whereas for most other diseases no one really cares. I think getting diagnosed with HIV also encourages some people to grow up a little bit in a way that getting diagnosed with high blood pressure or whatever doesn't, though maybe less so now that infection vectors are changing.