It's because she's the perfect oyaji/otaku type, with the added bonus of being enclosed in a lolicious girl's body. Add to that the archetypal tsundere[*] Kagamin, and you have a pairing that's really easy to add subtexts to, with all moe qualities you could ever want.
[*] Yes, I know, but there's still no better term.
I watched the anime. I found the girl to be pretty interesting and likable but still I don't know why she's so popular to the point she is to many people.
