Because ... you know ... the oxygen that isn't there? Well, it's being consumed ... by the tremendous number of microorganisms ... which feed on the oxygen ... which isn't there.
Anyway ... if the water were colder ... there would be more oxygen ... because ... science.
The reason that the warship “Vasa” did not decompose is because there are no ship worms in the Baltic to eat the wood. This is because they live in low oxygen/very salty water
The lack of oxygen in the water there is a phenomenon that is well known for decades. The reason is that the Baltic is a sort of sack where water goes in from the Atlantic but can’t get out. It is saltier than the Atlantic too for the same reason