Why do some people get aids from blood transfusions and some don't?

Untested blood. Nearly all of the people who developed aids from blood transfusions contracted them from blood that was not tested for HIV prior to 1985. Today all blood that is donated for blood transfusion is tested for HIV so there is very little risk of developing HIV from a blood transfusion. Cdc estimates risk to be approx 1 in 1.5 million. If you receive HIV + blood, 95% likely you will contract it.