Probably political corruption for developing countries. If the country hasn't signed a treaty/covenant, or they've signed one but haven't ratified it, there are also two reasons. There are even some law in Australia that have been ratified (from the human rights declaration) that aren't in power e.g. right to marriage.. gay rights not allowed.
May of strayed off topic but if you're lost, message me!
This is the correct answer. Ultimately, the number one barrier for countries enforcing human rights is state sovereingty. The government can pick and choose what rights they want to apply. Say a developing country has a booming child labour force. As long as if they have law acknowledging that they have the intention to break human rights or have not signed the necessary instrument (in this Case CROC), the United Nations are powerless to do anything. This is because Human Rights are very difficult to enforce.