Why do some countries have a high death rate when other countries have a low death rate?

For lots of reasons. They may not have enough food. Diseases are more widespread. They don't have as good a health service as richer countries. Living conditions are not good, so they may not have clean water and the standards of hygiene are not as good. Many poor countries experience famine. Many poor countries are not as well equipped to deal with natural disasters, so they suffer more when they happen. For all of these reasons and many others, the death rate is higher and life expectancy is lower in poor countries than in richer countries.