Why mosquitos cant transmit AIDS?

From: CDC National AIDS Clearinghouse (centre for disease control US)

From the onset of the HIV epidemic, there has been concern abouttransmission of the virus by biting and blood-suckinginsects. However, studies conducted by researchers at CDC andelsewhere have shown no evidence of HIV transmission throughinsects--even in areas where there are many cases of AIDS and largepopulations of insects such as mosquitoes. Lack of such outbreaks,despite intense efforts to detect them, supports the conclusion thatHIV is not transmitted by insects.
The results of experiments and observations of insect biting behaviorindiciate that when an insect bites a person, it does not inject itsown or a previous victim's blood into the new victim. Rather, itinjects saliva. Such diseases as yellow fever and malaria aretransmitted through the saliva of specific species ofmosquitoes. However, HIV lives for only a short time inside an insectand, unlike organisms that are transmitted via insect bites, HIV doesnot reproduce (and, therefore, cannot survive) in insects. Thus, evenif the virus enters a mosquito or another sucking or biting insect,the insect does not become infected and cannot transmit HIV to thenext human it feeds on or bites.
There is also no reason to fear that a biting or blood-sucking insect,such as a mosquito, could transmit HIV from one person to anotherthrough HIV-infected blood left on its mouth parts. Two factorscombine to make infection by this route extremely unlikely-- first,infected people do not have constant, high levels of HIV in theirbloodstreams and, second, insect mouth parts do not retain largeamounts of blood on their surfaces. Further, scientists who studyinsects have determined that biting insects normally do not travelfrom one person to the next immediately after ingesting blood.