Why don't police officers ticket drivers who hit pedestrians? Why do some drivers who kill pedestrians escape criminal charges?

You may be conflating two issues: citations for traffic infractions and criminal charges.

It is within the discretion of the authorities whether to charge a driver criminally in a pedestrian accident death. Typically it happens if the driver was intoxicated, on a cell phone, drag racing, or was reckless.

Not all "wrongs" or all injuries are crimes. However, I understand your question and concern. To be cited for a crime or a arrested, there must be more than just negligence. There must be at least a mental state of reckless disregard for the safety of others. This is often missing. Plus, the pedestrian is not without a remedy. When negligence is the issue, a civil case is certainly possible and insurance policies are usually a source of money for the injured pedestrian(s).

Here's a plausible scenario: A driver at night, generally obeying the speed limit and driving safely for traffic conditions, unexpectedly collides with a pedestrian who is drunkenly jay-walking, wearing headphones, and wearing dark clothes as he crosses the unlit highway on a curve. The driver sees the pedestrian at the last second, and brakes hard and swerves, but too late to avoid a collision. The pedestrian is severely injured, but survives. Would it make sense for the driver of the car to be charges with a crime? In fact, California law allows authorities to prosecute drivers criminally even in cases where a death is caused without gross negligence. If the driver was sober and not reckless, that is a misdemeanor. But you get the idea, hopefully.

In addition to everything my colleagues have already said, consider that the DA won't charge without witnesses, etc. If a person is sober and not on their phone and it's a true accident, a civil but not criminal case may well be most appropriate. There's no rule that every accident of type X must always result in charges; that's not fair or efficient administration of justice.

Indeed every situation is unique, but under California Civil laws drivers are bound without contract to abstain from injuring another person or damaging their property. The person may not be cited for injuring or killing the person, but may be held liable for the individual's losses. The difference between civil and criminal actions is basically intent. Most drivers do not intend to hurt someone else when they get in a car crash, however, when they are reckless drunk drivers, they speed or talk on a cell phone, they are in fact engaging in risky behavior known to cause a car accident. A stupid driver's mistake, such as not seeing a pedestrian, is not considered something criminal and therefore the driver may not be cited or charged. That said, it depends on the unique circumstances of an individual case and the parties involved, victims, witnesses and police. I hope this is helpful.