HIV is like a rootkit on the software of your immune system: it destroys the very cells that would be sent to destroy it. That's why it's so hard to vaccinate against. Fortunately, we have very effective treatments to suppress it and we can, if we deploy these widely enough, expect to suppress it out of transmission in the foreseeable future. HIV isn't very easy to transmit so if you suppress it in the people who have it, it should die out when the oldest person with HIV dies of other causes, after spending their life with suppressed HIV.

Herpes is a sneaky bastard that hides out in nerve cells, out of the way of the immune system. The immune system can deal with it if it can find it, so that's why it hides.

Note that the work to be able to create a Covid vaccine has been in progress for decades. BioNTech (the actual inventors of the "Pfizer Covid vaccine") was founded 15 years ago and the more fundamental research on mRNA vaccines was done before the company was founded.

A huge amount of this understanding of viruses comes from the ability to sequence genomes. This is almost magical - it really is science fiction come to life. It's as magical as the "Star Trek communicator" becoming "cellphone in your pocket, works worldwide". The Human Genome Project required rooms full of thousands of expensive machines working for several years to sequence one genome. Today, you can do a full sequence of a human-size genome in less than a day (current record: 5 hours 22 minutes!). You can grab a random virus or bacterium and sequence it overnight, just to see what's interesting in its genome. If you find another one tomorrow, you can compare them over the weekend. This is a huge change for all life sciences.

It was sheer luck that mRNA technology was almost advanced enough to make vaccines for coronaviruses when SARS-CoV-2 (the virus that causes COVID-19) showed up. A lot of researchers worked a lot of overtime to turn "almost" into "actual" in an amazingly short time.

We also have a lot of experience making vaccines to viruses that the immune system can handle when suitably primed (i.e. NOT retroviruses like HIV, but most others) in other ways - using similar virus strains, culturing viruses and inactivating them, weakened live viruses, and so on. These methods had been used to produce vaccines for the SARS virus (the vaccines were not widely tested because by the time they were ready, there were too few cases to test and no need for them). The same techniques were also used to produce SARS-CoV-2 vaccines.

We can expect a lot more vaccines for diseases previously considered impossible to vaccinate against, using mRNA technology.

Aside from mRNA tech, research continues on other diseases. RSV (Respiratory syncitial virus) is a serious disease for infants, but attempts to make vaccines were disastrous in the past and the reason why the first attempts failed has only recently been understood after 30 years of work. This work doesn't even use mRNA knowledge, it's completely separate innovation.

Virology and immunology are both extremely complex topics which we have, across the world, not nearly mastered. We do not understand either, not even nearly, so the rate of advance of knowledge is rapid in both. When they intersect, they are both even more complicated. You have to see the current state of vaccines as a work in progress, where some problems have been solved and others have not, depending on random chance and whatever seems most important at the time.

I'm nearly 50 years old. In my lifetime, we have gone from commonplace vaccines for only a few things - polio, etc - to vaccines for a whole array of nasty diseases (measles, mumps, rubella, diptheria, typhoid, tetanus, pertussis, pneumococcus, influenza (moving target, alas), etc).

I expect in 50 more years, we'll have zapped most of them.