As Jon Lin said in the comment, the serialization got canceled after a long break.
According to Japanese Wikipedia, the manga was serialized in Dengeki G's magazine since edition 2005/11 and it continued until edition 2007/2. On the edition 2007/4, the table of contents listed the series as on break:
Comic "Strawberry Panic" is on break for a while.
However, since there's no sign of continuation for a long time, the serialization got canceled instead.
As for the reason behind the break/long hiatus, it's not stated anywhere (it's a common case). However, I doubt it's "because Sakurako Kimino was sick" since:
- The light novel has finished (Sep 2005) before the manga serialization (Nov 2005), and the manga follows the light novel quite faithfully
- The mangaka is Takuminamuchi, not Sakurako Kimino, and the story material already existed
- Sakurako Kimino still continued writing: Baby Princess (2007-2012) and Love Live! (2010-)