1)Because it's popular or rated in the top 10 on most lists of pinball machines. I don't mean your own personal lists, but retrospective lists of pinball machines it's usually near the top. (Like if you asked anyone who is a good looking man? Many would say: Brad Pitt or George Clooney. People will say George Clooney is popular/hansom, women like him he's charming, finally it catches on he's popular because people think he's popular, he's well known etc. Some other man could be equally as good looking but not well known and we wouldn't know because he's not well known.). Similarly if you ask who were/are the most beautiful women people might say Kate Upton or Marilyn Monroe. Meaning people who are or were very popular tend to get mentioned more often.
2) it's got a theme that's not too serious, kind of cool and kind of nerdy, I mean you can like the theme and not be a total nerd. But I suspect a lot of pinheads are somewhat nerdy. It doesn't sound like it would be totally bad as a family game (like some might say South Park or ACDC might be inappropriate).
3) it's funny, has good to great art, toys and call outs, seems to have acceptable rules. I don't hear people saying it's super easy or super hard. It sounds like regular people can play it and like it and that real pinball people can also play it and like it.
I'm not sure that is all 100% accurate, but that's what I am kind of getting from what people are saying.
I have seen a few posts that say something like "It's not one of the better pins" but then they don't say why.
It's like those people just want to be contrary. If I said something like: "I enjoy a nice warm spring day sunny and 65 degrees, nice to go out and take a walk". And then a few guys would chirp in "Sunny Spring days suck" But they don't say why. So to me that's just sour grapes. If they would like to add to the discussion fine. Like I said I'd like to hear both sides of the coin. But just to say something like "it's not a good game" doesn't tell me much.