Location, vulnerablty. Your poop is filled with e.coli, yeast and thousands of other germs that hitch hike through your bowel. As long as you make an effort to keep them away from your vulnerable areas (wipe front to back) you can stay healthy. The female genitals are constantly moist, creating the basic moisture that aids germ growth. Wipe B/F and you allow microscopic patches of germs the chance to move into them.
Females and UTIs. Because there are more bacteria near your rectum and females have a very short urethra. Wiping in the manner you describe brings bacteria into contact with your urethra and increases the chance of getting a urinary tract infection.