It's because it's so short.
This series was just criminally short, but it was so refreshing to see a story that you know was fully fleshed out before a single panel was even drawn. The author had a vision, and completed the vision with momentum and expertise.
Every chapter was told as a normal romance would have been told. The thing that makes this stand out is the lack of silly tropes and misunderstandings. It was short, sweet, to the point, and beautiful.
I wish it was longer, but at the same time I don't. Both options just feel like paradise to me..
