Whether the immune system can beat it by itself if you survive it.

All the diseases we aren't to create a vaccine for (HIV, herpes, etc.) are diseases that your immune system is unable to beat.

For covid-19, your immune system will learn to beat it in a few weeks without a vaccine if you survive it. For those other diseases, if you have them, your immune system will never remove them from your body even if it has years to do so.

If a disease cannot be beaten with years of trying to learn how to beat it, it's unlikely than giving it a preview (which a vaccine is) will help.

It's because the viruses mutate fast and can hide from the immune system. For herpes, it hides inside neurons. For HIV, it hides inside CD4 T cells. The immune system isn't good at targeting neurons and T cells.

It's not because HIV attacks the immune system. It takes more than 10 years for HIV to destroy the immune system without treatment and it never does with treatment. The immune system is unable to beat it even when it's working well.