Short answer: there's no political capital to be made from renaming the Hoover FBI Building.

Hoover was a nationally acclaimed figure in his era. But, he's been dead for almost 50 years. Unlike our Presidents, whose names and bios appear in textbooks in every school in America, Hoover's name and bio don't. Most people today don't know who he was, and those wo do, remember him as the nation's top cop, the bulldog-visaged G-Man, the scourge of commies and bank robbers--not the bureaucrat who harassed Martin Luther King, deliberately undermined antiwar, women's lib and civil right movements, placed illegal wiretaps, and kept voluminous files on the peccadilos of politicians and public figures. So, while some local Board of Ed officials score political points with some constituents by renaming schools because George Washington and Thomas Jefferson held slaves, or because Lincoln was a bigot, or because Woodrow Wilson re-segregated the Civil Service, nobody sees any political points from renaming the Hoover Building. And, it'd further demoralize the FBI. It's a political sleeping dog--let it lie.

For the record: Hoover did a lot of his dirty work at the behest of Presidents, who used the dirt he dug up against their political foes. He also turned a corrupt federal agency into a first-class investigative and law enforement body, and created a world reknowned crime lab and fingerprint and criminal records archive system that are widely used by state and local police agencies.