Reservoir Dogs has two credited music supervisors plus one credited assistant music supervisor. It’s possible their work was limited to obtaining clearance (licensing) for songs chosen by Tarantino, but it’s also possible if not likely they participated in song selection (maybe not the assistant).
One thing music supervisors often bring to the table is a comprehensive knowledge of extant music. Along with that, most music lovers have songs, albums, bands, and genres that they love but which were never top hits. Both directors like Tarantino and music supervisors can be motivated to explore their musical knowledge and collections for music that they understand, feels right for the moment, but isn’t overplayed. Lately a lot of “summer blockbuster” movies seem to use the same cues for the same situations. Auteurs like Tarantino want to have a more original and particular voice and tone in their films, so they are just as likely to place obscure tracks in their films as hits.