Because the security of Diego Garcia is that much easier to enforce when only the people you have vetted are allowed to be there. If no one lives on the islands, then any unidentified boat is an obvious security threat. But with the islands inhabited, that boat could just be a local fisherman slightly off course.
Also, it's a lot easier to do sketchy shit in your top secret military base in the middle of the ocean if there's no one within ~1000 miles that isn't already involved.
Was at a brewery last night where the "bathroom" was a row of sinks, and then a row of doors labeled "sit" or "stand". The commode cubicles were full length walls and doors, totally private, and this just makes so much more sense to me