Being a left-winger certainly doesn't hurt him, especially since people love it when high-profile individuals agree with their political opinions (hence why basically every celebrity not known for being conservative has come out to shill for the Democrats or what-have-you).
But I think the biggest thing behind Fantano's audience is that he appears to combine the best of both worlds regarding expertise: he's not an industry insider who will spend half the video sperging about the production process and the history of music (like your average video essayist) but he has reviewed so many different genres of music over the years that he gives the impression of being very knowledgeable and reliable.
The fact that his numbers don't necessarily always line up with what you might see on Pitchfork etc., and that he is pretty wary about giving scores over about a 7 further makes people think of him as a rational and sensible listener whose opinions aren't swayed by current trends. As a result, his tard fanbase feels more comfortable using him as the basis for their music taste. (That is, I have seen people comment that they like their favorite albums/songs/artist less after he gave them a lower rating, lol)