Yeah, apparently the manga was cancelled while the anime was airing, likely due to poor sales and reception, so they just had to end it with the chapters remaining until they could close off another volume and that was it.
The 4koma's second volume was just outright cancelled along with the web version, so only half of the strips were ever printed (along with a few in the third manga volume to pad it out a bit). Well, at least Midori's still doing stuff, she's drawing Ninja Soccer now and I sure hope for some background cameos.
Lucamarte, the general opinion is that the anime went too far into the Darth Yura plotline which sort of made it "dark", though not really in a "grimdark" sense.
Commentator, the personalities were pretty much set in stone in the prequel manga already so they worked off that in this manga, but the anime went another way indeed.
Speaking of which, while it's a pain to scan the middle school volumes and hardly interesting to many (it's a weird 4koma + normal manga chapters format, too), there is that crossover with Anime Tenchou also hosted here that can give you a taste for the prequel despite being more silly than anything.
