because she was saved by Wesker ! You can get more detail information by reading his report. Quote : "...My instincts sensed danger, something had to be done, quickly. My instincts did not disappoint me. Even though Ada almost had her hands on the G-Virus, which Leon had acquired from Sherry, that affection of hers drove her to her death. But she was still of some use. I had to save her life. My people hurried to retrieve the G-Virus that Leon threw away. But Hunk, the only survivor of Umbrella's salvage team, was there before us..."