Well, let me guide you down the road of understanding, my son.
I guess you could say there's a number of reasons.
K-On! began airing in 2009 and it was undeniably a popular series for all the reasons that you can imagine. Now, as fanbases of shows are want to do, they began to proselytize to the masses about the wonders of K-On!. They'd do deploy the usual internet weaponry in the form of forum avatars and gifs of the show in an attempt to spread awareness and celebrate their love of the property. Now this creates a few problems.
Firstly, we get the usual reaction from the internet when they've been bombarded by any fanbase: shut the hell up and stop spreading meme's everywhere. That's the usual 'tired of group' response which happens even when the show is very popular see: Tengen Toppa Gurren Lagann.
Secondly, we get the MLP reaction where some people can't handle people enjoying a series that is primarily made for a female girl audience. As you may have noticed, this really causes people to lose their shit for reasons that are kind of beyond me. Anyway it's a well documented phenomenon.
Thirdly we've got to deal with the concept of moe. Remember that by 2009 anime was seen as being in a slump with the glorious of heydays of 2007 quickly forgotten. The general conesus being that anime used to be cool and now it's just a lot of shitty, pedo, pandering moe shows for otaku. How disgusting. Then here comes a 'moe series' (I hate using this term incorrectly but then again everyone uses this term incorrectly because no-one can decide what it means) which has a large and vocal fanbase. They appear to represent some kind of new movement that loves 'moe shit', which means that all the people who grew up watching 80's/90's anime are totally free to hate on this group because they love the shit that anime has 'turned into' (in their mind). This reaction, crucially, doesn't really even deal with the worth or quality of the series itself because it's completely irrelevant to what the series represents e.g. the downfall of anime.
Finally I'd say that anywork which has people shouting it's praises to high heaven will inevitably draw criticism when people watch it for themselves and perhaps find the work extremely boring. There's then a great divide between the people who are calling it the greatest thing since sliced bread and the people who think that it's the cure for insomnia.
