Why does it take longer to be promoted in the Marines?

The Marines are probably the slowest to promote in the entire Military. I am thinking about making a career out of the Marines. A lot of people say a career in the Military is better in the Army since they promote much faster. I want to be a MARINE so this doesn't apply to me. I was wondering why do the Marines take so long to make a promotion? I know that lower ranks have more responsibilities than any other branch, but that can't be the ONLY reason why the take longer to promote.