Just want to clarify; the list was made in 2004 so if you're wondering why some cases were omitted, that might explain it.
I believe the list was simply an account of the biggest/most well known/most famous unsolved cases in the past 150 years. Almost all the cases listed have a multitude of information about them on the Internet, there's been lots of books written about them, movies and TV specials made, and a lot of them retain name recognition amongst the general public. Cases like the Sodder Family, McStay Family, Springfield 3, Oklahoma Girl Scouts, Keddie Cabin murders are all very intriguing cases, and they are complete mysteries, will never make a list like this because they aren't well known outside of their local areas and forums like WS. I wouldn't be surprised if the author of that article had never even heard of those cases. The same cases always make these "Top Unsolved Cases" list: Ripper, Dahlia, Borden, Ramsey, Zodiac, Cooper, Earhart, etc.