HIV is hard to make a vaccine for because without consistent treatment there is no way for the body to fully eradicate it, and it will only progressively get worse. Even with medication, it only suppresses the virus. The body is unable to replicate this protection on its own without the assistance of medication. If a vaccine existed that could stimulate the immune system to produce this defence naturally, it wouldn't have to rely on medication, at least not entirely.

So yes its hard to make a vaccine for something not fully understood and why the immune system cant develop any form of natural resistance. No matter how long someone has been diagnosed as positive, their immune system has no ability to ever learn to protect itself from this.

because covid has/had no barriers to who or how it infected the general population and how easily and quickly it spread. HIV doesn't have the same impact to the world population as a whole.