There isn't a specific reason for it. Different stutterers struggle with different sounds. No one is really sure why. The actual root cause of suttering hasn't even been discovered with 100% certainty. Things that affect the brain tend to manifest in different ways in different people. Brain chemistry is very complex like that. Research papers also don't really focus on specific sounds that stutterers have issues with apart from speech therapy techniques that may work for those specific sounds. They tend to focus on the cause of stuttering in general.