It's in Weekly Shonen Magazine, so the demographic is the same for Fire Force, Runway de Waratte, Hajime no Ippo, and Bakemonogatari. The series is quite popular in Japan. 2 million copies across 10 volumes, and 3 million by the 13th volume. 200k per tank usually rates something as very popular. Extremely popular for not being in WSJ or Hana to Yume.
I think what makes most readers and viewers angry is that see parts of themselves in Kazuya that they don't want to admit are there...
He barely moved in his apartment after Mami dumped him, and before he rented Chizuru. He's probably prone to depression, because that's what it looks like. Same with his behavior on the beach. We know he has low self-esteem.
He's not a role model, and he's doesn't feed escapism. Then again, he's entwined with a cute girl with a fiery personality, a killer body. So it does and it doesn't.
I like the series because the characters have depth and react in generally realistic ways.
Edit: I guess it hit 5 million this week.