Regarding the IMDB rating, that’s (at time of writing) an average of 69,953 user ratings. So, quite simply, about 70,000 people really like it, and rated it on IMDB.
As to why IMDB users like it, it’s a bit speculative, but IMDB is popular within the movie and TV production community (as people who work in the industry can use it, to some extent, to promote themselves). Twin Peaks may appeal to them because it includes elements that satirise television itself.
The multitude of characters you refer to, along with the melodramatic tone and certain plot elements of the show (Identical cousins! Doe-eyed teenagers in love! Murky local business dealings! Almost every married person having an affair!), and the fact that it’s a serialised and continuous story, are meant to imitate and satirise American TV soap operas. You may have noticed that the show even includes its own fictional soap opera, Invitation to Love.
Twin Peaks was partially intended as a dark and disturbing parody of soap operas, to fit with Lynch’s perennial theme of revealing the darkness lurking underneath the bland and glossy surface of suburban America. Being a parody of a much-looked-down-on genre of TV, whilst having very high production values, may appeal to people who work in the industry, as they’re primed to be aware of the aspects of soap operas being parodied.
Apart from that (and unfortunately I can’t find the interview where I read this tidbit right now), I believe Lynch has discussed how he received effusive letters from survivors of domestic abuse after the second season aired, as dark events happening in suburbia aren’t often shown on mainstream American TV. (One such fan Kickstarted a Twin Peaks documentary about his experiences.)
I don’t think that has much bearing on the general high esteem in which the show is held, but it’s an interesting point about how fictional works can resonate very strongly with some people, whilst passing others by entirely.