Generally Editor takes decision on a Manuscript based on minimum two reviewers comments. So one possible reason is the required reviews are not completed yet. Followings are few points which are responsible for it.
- Editor has contacted many suitable and potential reviewers, but at least two of them has not agreed to act as a reviewer of your manuscript.
- Minimum two reviewers have agreed, but one or both of them are delaying to submit their comments
- Sometimes, editor does not satisfy with the one or both reviewers comments, so again he/she send your manuscript to another reviewers.
So there are many things are in behind. But most of the journals generally takes 2-4 months for the first decision and some journal takes more than 6 months and some even more than 1 year depending on above situations.. So if you are in hurry to get this publication and can't wait more, then you may request to the Editor for a final decision at the earliest possible, otherwise you are going to withdraw your manuscript. Since there are number of suitable journal available, so you will definitely find another one.