There are actually many reason to why the ship where so well kept after 333 years in the water.
One of the reasons are the polluted water and another is the low oxygen content in the Baltic sea.
The main reason why Vasa is so well kept is the fact that the ship sank whole and in brackish water. (Brackish water is a mix of salt and sweet water.)
Brackish water does not have shipworm, which would otherwise have cause great damage to the wood. Shipworm eats wood that is in the water.
We have many shipwrecks in Swedish water, but only one that was completely whole when it sank, and that is why we still have 98 % of Vasa left to show our visitors at the Vasa Museum.