Q: Why do journals take too long to decide whether the research is appropriate and convey rejection?

Most of the journals take up to four weeks for the peer review process. This time varies across different journals. Some journals indicate the time from submission to first decision and the time from submission to final decision on their websites.

Lengthy peer review times can be indicative of non-availability of appropriate reviewers for a particular manuscript. Usually, initial editorial assessment is done within a short span of time and a decision of whether or not the manuscript fits the aims and scope of the journal will be decided and conveyed to the authors at the earliest. Sometimes, this initial evaluation may be delayed due to heavy inflow of manuscript to a journal or non-availability of editors at a particular point of time.

Hence, it is always better to write to the journal editor after about 3-4 weeks of submission to know the status of a manuscript. In your case, since the manuscript was assigned to reviewers, it is possible that reviewers may have declined to review your manuscript or they may have felt that the manuscript is not suitable for the journal readership. In any case, it is a good idea to follow-up with the journal when the status does not change for a long time. This will help you to withdraw your manuscript to submit it to another journal and it will save time. 

Suggested reading:

By when can I expect reviewer comments if the average review time for the journal is 75 days?

How long should I wait for a response from the journal?

Should I send a reminder if my paper is in peer review for 5 months?

Where can I find the time taken from submission to acceptance for a journal?