@ngkn92 By the time you start sucking, the venom is already in the bloodstream apparently. Sucking won't do enough to make a difference and will increase the chances of infection.
http://mentalfloss.com/article/90886/can-you-suck-poison-out-snakebite
The fact that people survived in the past using the suck method seems to be attributed to the variance in the amount of venom that the snake decides to give.