Well, I would say because it's too long... but, then I spent the last 5+ years trying to watch EVERY Doctor Who episode/special to ever exist that I could legitimately get my hands on, so I can't fully fault the Naruto series on that alone... Then again, like most series that have lasted this long already and is still going to this day, there are going to be more glaring hits and/or misses moments than there were beforehand... The fillers, for example, are a glaring miss while certain battles, if you only watch then in a YouTube-style compilation without any interruptions, are a glaring hit... sometimes...
Now, it more comes down to either a subjective different taste as you grow older and find different anime series to pass your time or an objective dislike to how this particular series has and is being handled to this day... Either you stop watching and/or reading Naruto because you've found other anime/manga series that, in your opinion, are better than Naruto or you stop watching it because you grown tired of how the series is being handled...
More or less, the "hatred" this series gets alongside other series that are grouped alongside with Naruto (like Bleach and One Piece) comes from both the subjective and the objective point of view... You can choose to listen to them or not, it's all up to you... For me, I have no strong feeling one way or another...
Now, it more comes down to either a subjective different taste as you grow older and find different anime series to pass your time or an objective dislike to how this particular series has and is being handled to this day... Either you stop watching and/or reading Naruto because you've found other anime/manga series that, in your opinion, are better than Naruto or you stop watching it because you grown tired of how the series is being handled...
More or less, the "hatred" this series gets alongside other series that are grouped alongside with Naruto (like Bleach and One Piece) comes from both the subjective and the objective point of view... You can choose to listen to them or not, it's all up to you... For me, I have no strong feeling one way or another...