Native swede from Stockholm here.
Supposedly it is because very few ships from the 17th century are so well-preserved. For many decades the museum was just the ship itself, nothing else. You'd probably have to be quite a bit of a nautical nerd to find that alone interesting for more than 10 minutes.
But in the late 80s/early 90s they built a proper museum around the ship to give it more historical context. So what the museum is really about is the 17th century Sweden. Sweden was a powerful military nation at the time, and in war with half of Europe during The Thirty Years War for which the ship was built. Apparently Sweden was something of a historical extreme during this time period, spending pretty much the whole national budget on the military only.
If you aren't too interested in history but perhaps human nature, the ship's story is also one of megalomania. The King wanted to build the biggest and most powerful ship in the world, a major undertaking. The shipbuilding expert who was tasked with leading the job soon realized that it wouldn't be feasible to make the ship as big, with as many canons as the King wanted. However, nobody really dared to speak against the King, so they built it anyway. And so it sank during the first "live test". Really just because the King had insisted on making it so big, but of course nobody dared to blame the King for the fiasco.
It is a great museum if you are interested in history in general, but it is also very tourist-friendly. There are exhibitions re-creating the interior of the ship and the life on board, and you will be able to view the ship from every angle but obviously not enter the ship itself since it is too fragile and valuable.
Stockholm is one of the cities with most museums in the world, and a lot of them are scattered around the area where you find the Vasa museum, located on the island Djurgården, most of it is a big park area. Nearby within walking distance you'll also find various other museums, such as the Nordic museum (also historical, only recommended if you are indeed interested in history) and Skansen, which was originally the city zoo, but is nowadays a mix of a zoo with focus on nordic animals and an exhibition of old buildings/history (good place to visit with kids). Nearby you also have the amusement park Gröna Lund.
As a tourist I'd strongly recommend going to Djurgården by boat from Old Town (the station is called Slussen, you can get there by subway). The boat is using the same tickets as used for bus or subway. Alternatively you can walk there but it's some 2-3km from the city center, or go there by tram.
One thing you should know however, is that they sell the Stockholm visit by having all tourists visit Old Town and Djurgården, which isn't wrong in itself, but consequently it gets crowded in those areas all year round, with expensive prices.