As I recall that was a list of the most re-watched episodes, which is not quite the same thing as being the most watched show. It may be that people are inclined to do more comprehensive re-watches of the other series, for instance because of the greater serialisation of DS9 or more consistent quality, whereas they would tend to just pick out a few favourites from Voyager.
The way I understand it in regards to Netflix, TOS and TNG are popular, while DS9 and VOY get more repeat customers.