Because the vaccines aren't 100% effective. They're least effective on most vulnerable. It's not proven that vaccines don't cause autism, it's just that every valid and reliable observation showed no relationship, and they do have other side effects. But Measles was wiped out in the most economically developed countries 20 years ago. Now look.

That minor disease killed 144,000 people last year. It also causes blindness, deafness, profound physical disability and so on.

Because vaccination only works about 90% of the time. The kids most vulnerable to catching infectious diseases are the ones least likely to develop the anti-bodies. Therefore Anti-Vaxxers = random infanticiders.