Why did the Strawberry Panic manga end?
Generally, a manga/anime doesn't have to complete the story set out by the original material it was adapted from. Some examples are Accel World, Bleach, and Bodacious Space Pirates.

Some reasons are obvious, like key changes in the plot making it impossible to follow the original plotline set out in the source, while other times it's not so obvious such as events involving members of the production team, such as work disagreements, licensing problems, or people getting sick for an extended period of time.

This isn't just for adapted works as these problems can affect the source material: Kaze no Stigma's light novel has been left incomplete due to the author dying.