Editor rejected paper because of insufficient reviews
It seems to me that the editor asked people to review your article, but not enough people were willing to do so. Apparently, the editor thought your article had enough of a chance not to give you a desk-reject, but your article (or your abstract) was not exciting enough to convince potential reviewers to review it. 

Maybe it is not the article/abstract, but your article is dealing with a very narrow subject, thus limiting the potential reviewers. You had the bad luck that all of those few potential reviewers were all busy. 

I don't think there is much you can do, other than have a good look at the abstract, and see if it is excessively boring. If that is not the case, then you probably just had bad luck.