Why is Drupal Core not updating via composer?
After running composer why-not drupal/core:8.5.4 and doing some more research on the error message, I came across Upgrading from 8.5.0 to 8.5.1 using composer

This command did the trick for me. 

composer require drupal/core:8.5.4  webflo/drupal-core-require-dev --update-with-dependencies