I've seen the same question on other forums, but have not come across any definitive explanation. The non-cited answers seem to fit into three categories:
Perhaps it has to do with a combination of these three, or none of them at all. Some have even said that with practice they have been able to raise both independently from the other. Why is this so?
The muscle responsible for being able to raise both eyebrows is one and the same (m. occipitofrontalis). You can not raise one eyebrow because the whole muscle is connected to both eyes. It is possible that one side is not connected or less connected but that's usually not the case and such a person would not be able to raise both eyebrows at will.
What happens in almost all people is that they are not raising one eyebrow, but instead keeping one eyebrow down and raising both at the same time thus giving the impression of only raising one. There are various muscles that are able to lower the eyebrows.
So if one person is "not able" to raise one eyebrow, it actually means that they are not able to keep the other side down.
