Why did the Strawberry Panic manga end? I picked the so-called 'complete collection' of Strawberry Panic at a convention. Upon reading it however, I found the story wasn't actually complete (stops just as the 'faceless devil' event is announced) and closes with an "..end?" even though nearly no plot points are resolved and many events are about happen

I've read the light novels, and there's a good chunk of story left. Why was the series stopped so suddenly?
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.