I will try to keep the explanation simple. Firstly, the scientists were able to correct the genes of ill cells extracted from 6 children affected by these diseases, but this was not enough. To re-introduce the healed cells into the human body can be extremely risky, therefore the need of using a virus like HIV. They got rid from all the negative aspects of the virus, while they kept just the only positive one, which is its ability to modify the genes of human cells. In the opinion of Luigi Nandini, the coordinator of the therapy-team, the use of the 'cleaned-up' HIV virus was they key factor of the success of the research - all the previous attempts had not given serious results.
Maybe this kind of copy-and-paste technique with human genes can sound strange, we are talking about the human body. It is also true that when you are able to take advantage from something like the HIV virus - the cause of a plague at a world-wide level - it means that we were given an extraordinary power that can do really good. The real problem is not just "what you do", it's "who does what". It's our responsibility. This is also why this group of researchers is going to publish every detail of the work they have done - and they are not afraid of doing it - since transparency and the respect of rules are the main principles in this field of research.
Source: Designers Against AIDS website