I think people are misunderstanding something. Yes it sucks when the series ends early, but it depends on the popularity of the adaptation, mainly in Japan.
Some novels treat it like the ad festival promo that it is. It sucks, but they know the qual of their work. Others that are super popular know they have a shot, and so do the publishers. They put more money behind it, and it blows up enough to become a full series.
It’s the same principle for some anime. Although it’s easier to tell with anime, as they use cookie cutter, bare bones animation and designs. Also they butcher the story to get to the “good” parts of a series faster. Off the top of my head, I’d say Otome mob, Mato Slave, S-Class Daughter, hell, Air Gear all fall in this category.