Why couldn't Washington out law slavery at the state level?

[x]

Somebody told me the federal government was always the primary government when federal laws and state laws conflicted.

That depends on whether the law involves federal or state powers.

The distinction's gotten very murky over the years.

There are some areas where state law prevails, but a state law that sought to overturn federal regulations in an area where the federal government had authority would most likely be overturned by the courts, just as a federal law that usurped state powers would also be questioned by the courts.