Edward was dying of the Spanish Influenza; Carlisle was his doctor. There was no hope of Edward surviving, and Carlisle was slowly building a "family" unit; if anyone was to become a vampire, why not Edward, whose mortality was at an end anyway?
When Edward was 17, his family caught the Spanish Influenza. His father was already dead, and his mother's dying wish to Carlisle was that he do everything he could to save her precious Edward. So, when Edward had hours left, Carlisle bit him, turning him into a vampire. Edward was the first Cullen to be changed by Carlisle, followed by Esme, Rosalie, and Emmett. Alice and Jasper had already been changed, found each other (Alice knew that they were meant to be), then set off to find the Cullens. They became part of the family then. And so the Cullens became the vampires of Forks, Washinton.
Edward was dying of the Spanish Influenza and his mother knew she was going to die but she didn't want her son to die so she asked Carlisle to save Edward.
Carlisle converted Edward because his mother sort of pleaded with Carlisle to save him. She knew she was going to die and didn't want her son to share her fate. Carlisle said in the book when she last looked at him and asked him to save Edward it was almost like she knew what he was and he had to save them.
Edward and his Mother were both dying of "Spanish Influenza".
It was Elizabeth's Death Wish for Carlisle to do everything in his power to save her son Edward Anthony Masen. Edward had only minutes left so Carlisle bit Edward and let the venom spread and turned Edward into a Vampire. Elizabeth was Edward's Birth Mother.
carlisle turned edward into a vampire because edward was very sick
Carlisle converted Edward into a vampire because Edwar was about to die and that was the only way to save him.
edward and his mom had the spanish influenza and they were both about to die so edwards mom ( before she died) asked him to save edward no matter what it took so he did.