A DJ might refuse to host your mixtape because they don't want to be associated with the music on it. A DJ is a brand, and they want to be associated with good music only. DJs also have reputations to protect. If they start hosting terrible mixtapes, their reputation will go down, and people will stop booking them for gigs. DJs are also selective about the music they play because they want their sets to sound cohesive and well-produced. So if your mixtape doesn't have any good tracks, a DJ will probably refuse to host it.