Oh god. This is going to be a particularly obnoxious thread.
Anyways, I think the biggest reason is because Metroid doesn't have characters or faces that people can connect with. People really care about having a character like Link to self insert into, or a character like Zelda to connect with, and Metroid doesn't have that. Going further, Metroid doesn't even really deliver on the power fantasy elements in a way that is compelling to general audiences. In Metroid the power fantasy has to be earned, and while that's also true in a game like Zelda, your character isn't continuously put into an oppressive atmosphere in quite the same way. Zelda doesn't punch down as much thematically, the power fantasy is more innate.
I also think it's probably the case that people are more likely to quit Metroid than quit Zelda due to not knowing what to do next, even though both Metroid and Classic Zelda can have some cryptic puzzles. The backtracking probably makes it worse.