Because its real and warm in same time. I like its melancholic mood, i like family relationship, i like characters - complicated, strange but good peoples.
I can understand why its not popular - i skip it first time i seen poster. It look cheap and outdated but in same time i can tell what it have soul inside. I can compare it to old hand-made toy by which you pass in the store, but if you find it in the chest in the attic you can feel the love with which it was made. So just wait until you will be in mood to find this treasure.
Romantic parts not usual - if you like stuff with happy-pair you will disappointed. In same time parts about friendship and self-realization - the best.
So this show definitely not for entertainment, its more like warm cup of tea in cold autumn evening - you can feel it when you frozen.