At the time the Dutch had been getting the majority of the salt from Spain. However after the Dutch formed their own kingdom, partially from the Spanish Netherlands, they entered conflict with Spain and thus lost access to the salt they used. So they started making their own salt in the Caribbean to make up for it.

As for why they didn't make their own salt in the Netherlands, they do make/mine salt there, but nowhere near the quantities they consumed, so the loss of access to Spain's Mediterannean and Mined Salt hurt them. They don't have access to the constant sunlight that the Caribbean gets for making Solar Sea Salt, so they mine Rock Salt. At the time they figured out they could make salt in the Caribbean it was just easier to do and cheaper overall than mining. As a bonus they pretty much owned the salt trade into the colonies and even had enough left over to send back home.