The two quickest ways to drain blood from the body is through the femoral arteries and the jugular veins.
The femoral arteries run through the inner thigh and around the back of the knee.
The jugular veins are located toward the back of the neck, behind the spine, and in front of the spine, right behind your throat.
The jugular is much easier to reach and bite, and it's a lot quicker. A vampire can bite anywhere in the body, but a bite in many other places would most likely begin the healing process before the body is completely drained of blood, unless there's constant sucking. Even if there is constant sucking, it would take much longer to get all the blood.
Also biting the neck provides the vampire with a much better position from which to control a struggling victim. Much harder to hold on to a kicking screaming thrashing victim while biting the thigh or wrist. Vampires must be very practical if they don't want to miss out on dinner.
The jugular vein is there, and your body pumps a lot of blood into your brain to keep it going. It would also kill them quicker. That's why a lot of carnivorous animals also go for the neck when they attack a human.
Back when vampires first became the popular monster it used to be because there were more veins in one area. But now with modern culture, the biting of necks is yes more of a sex thing.
