Why should deepwater shrimp on different sides of the isthmus have diverged from each other earlier than shallow-water shrimp?

It is because the deep water shrimp and the shallow water shrimp being in different geographical location for a very long time have gotten use to their location and will definitely exhibit divergence.

Reproductive isolation occurs faster in deep-water shrimp than shallow-water shrimp.

Explanation:

Though in the same territory, the blockage caused by the isthmus would quickly and permanently isolate shrimps living in the deeper parts of the water, thus making them unable to breed. This situation would then caused lack of gene flow within the deep-water shrimps, and the emergence of new species that are genetically different (diverge) from one another

The shallow-water shrimp, on the other hand, experience minimal isolation due to the shallowness of water, and could still breed with one another. Thus, they experience a relatively lower reproductive isolation