It's really not an easy thing to find a suitable reviewer who is willing to do review and can make professional and to-the-point comments.
An invited reviewer wrote an email to us as follows:
................................
Due to conferences, fieldwork and personal matters I have no time for reviewing (nor my own office work!) until mid September. If you're still looking for reviewers for this paper then, please email me again.
Now it's only April, if we still can't find reviewers' in Mid Septemer, that means we haven't found the suitable revievers yet in the future 5 months. It will be a sad thing for both the author and the journal.
Some reviewers directly reject the review invitation, some decline the invitation in a more subtle way.
In order to hasten the manuscript processing, editors usually select several even tens of reviewers once!