There is no definite reason why the series is on hiatus. Daisuke Sato the series author, has been known to take extensive breaks from writing, so that could be a factor, but it is also likely there are multiple other reasons. The fact that Triage X, Shouji Sato's other series, has become a monthly series and is growing in popularity could be another factor. We can only speculate about the real reasons. All we know for sure is that the series will return eventually.