Only a small fraction of published manga is animated, and the selection is not simply the most popular titles. Several practical conditions have to be satisfied at once.
Sustained sales matter more than a strong start
Publishers look for a title that has held its readership across many volumes rather than one that opened well and faded.
An adaptation is commissioned years before it airs, so the relevant question is whether an audience will still exist at that point.
A series with a steady, committed readership is a safer basis for that judgement than one with a large but volatile one.
There has to be enough material
A season needs a substantial body of source material, both to fill the run and to leave the manga far enough ahead that it is not overtaken.
Catching up creates a real problem, since the animation cannot proceed without a story and inventing one risks contradicting the author later.
This is why adaptations of very successful but short series are often delayed until the source has accumulated enough volumes.
Some structures animate far more cheaply than others
Detailed backgrounds, large crowds, complex machinery and intricate costume designs all raise the cost of every frame in which they appear.
A series set largely in a small number of interiors with a modest cast is dramatically less expensive than one built on continuous spectacle.
Producers weigh this carefully, and it is a genuine reason some acclaimed titles remain unadapted while less celebrated ones are animated quickly.
The financing group has to be assembled
Production is typically funded by several companies contributing together and sharing the returns across their own businesses.
Each participant is looking for something specific, whether that is merchandise potential, music releases, game rights or distribution.
A series that offers little to most of those partners is difficult to fund even when it would be popular, because no one participant carries the whole cost.
The adaptation is often an advertisement
For a publisher, an animated series is among the most effective ways to increase sales of the source material, which continues long after the season ends.
This is why adaptations sometimes cover only the opening arcs and then stop, having achieved their purpose without needing to continue.
Readers experience that as an abandoned series, while for the publisher the campaign worked exactly as intended.