HPV strains 16 and 18 are the ones that cause the most cases of cervical cancer, thus it makes sense to vaccinate against them. It's just like haemophilus influenza Type B vaccince. Hib used to be #1 cause of neonatal meningitis. Countless babies died from it. Now it's extremely rare. Why? Because all kids are vaccinated against it (since 1992). The vaccince does not protect you from all strains of haemophilus influenza, only type B! But this was good enough since type B was the culprit here.