Given the ages of the perps it is likely they could not even conceive of actual punishment happening to them, much less death. Their time horizons are probably not much more than a couple of hours. Talking to them about consequences to acts that will occur next year or next week is like talking Euskara to your five-year-old. It is simply not intelligible.
Age is one part of it but IQ is the major issue. Many of these street thugs are operating with IQs in the 70-80 range. They're borderline retarded. Whether they're burglarizing homes, robbing liquor stores, carjacking drivers or mugging pedestrians, they act impulsively, like animals, putting no thought into either planning or getting away with their crimes. They simply seize the moment. Need money? Let's rob the next place we drive past. Their low IQs make them easy to apprehend often within hours, if not minutes, from the scene of the crime.
Unfortunately they are often needlessly violent, killing compliant victims when it makes absolutely no sense to do so. In many cases,they'd have gotten away with their property crimes had they not attracted greater attention from law enforcement by turning what should have been a simple robbery into murder.