While most viruses have a very dense conglomeration of structural proteins on the virion surface, essentially helping increase the integrity of the structure, HIV has a low amount of protein spikes on its envelope. This paper argues that by evolving a low density of these envelope protein spikes, there is prevention of immunoglobulin G antibodies binding to the virion, which in turn slows down the antibody response as well as the neutralizing antibody response. The latter has been known to be very slow to develop in case of HIV infection, and the low density of envelope proteins is thought to be the answer to this conundrum.
This information is particularly relevant when thinking about vaccine development for HIV, as this paper suggests using this knowledge to create a "virus-like display" vaccine that would generate neutralizing antibodies and as a result, an HIV prophylactic vaccine . I invite you to read more on the specifics at http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1004254