Why is AIDS not a congenital disease?
A congenital disorder or disease is a disorder that is present at birth.  This term can be applied to many conditions, including some that are infections.  More typically, the term is used for developmental birth defects such as spina bifida and cleft palate.

Although HIV/AIDS can be congenital (since a mother has about a 25% chance of passing it to her newborn), most cases of HIV/AIDS are not transmitted by pregnancy and birth, but rather by sexual contact between adults.  Hence it is inappropriate to call AIDS congenital since most cases do not occur in newborns.  This can be contrasted with, for instance, spina bifida and cleft palate, which essentially only occur in newborns and so can reasonably be called congenital disorders or diseases.

A specific case of HIV/AIDS, one which has occurred in a newborn, can be called congenital HIV/AIDS, but HIV/AIDS cannot, in general, be called congenital.