They couldn't do it at all until they had really good HIV-suppressing drugs. You won't do a transplant into somebody with an active HIV infection.
They still won't do it if they can avoid it. This an extreme measure, killing off the entire immune system and replacing it. They still need to find a donor who is sufficiently compatible, and then has the right set of genes.
So it just took time for all of the pieces to fall into place.