One band/singer whose popularity you will never understand.
Hmmm, a tricky prompt today. There are plenty of acts that I may not necessarily like, but I can understand the popularity of to their target audience. I am not a teeny bopper, so the oeuvre of Justin Bieber is not aimed at me, but who am I to cavil at those who do enjoy his squeaky clean auto-tuned offerings. Live and let live, I say.
However, for something that falls into the 'what the hell were they thinking' category, I present Bob Dylan's Christmas Album ... :-)