A wordy title
When NCIS was greenlit, producers were figuring out what the official name would be for the show. Initially, they decided to name it Navy NCIS, which came off as a bit too wordy. However, the meaning behind this was obvious.
CBS
CBS didn’t think audiences would know what the acronym meant so wanted to give more information in the name. At this point, it was essentially called Navy Naval Criminal Investigative Service. The name was redundant and they changed it.