Destroyers are around 8000 tons. Cruisers are around 10,000 tons.
You need 16,000 tons of metal to make 2x Destroyers. You only need 10,000 tons of metal to make 1x Cruiser.
Your crew sizes are also merged together with the larger-ships, meaning you need fewer sailors to perform the same amount of firepower. Two Destroyers would have a lot of redundant jobs compared to one Cruiser.
Why? Wouldn't two destroyers be more flexible and also harder to hit than one big ship?