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.

Based on my experience in academic publishing, I'm guessing that the editor invited multiple reviewers, most of whom declined but one agreed. The one who agreed wrote the review you received. The editor discerned that it's not particularly deep - as you write it's mostly cosmetic - and so was not willing to accept it based on the one review. However, since the reviewer had already written the review, the editor decided to share it anyway (they could equally have said "reject because insufficient reviews" and not sent the review report).

Another possibility is that a reviewer gave confidential information that led the editor to reject. This could be when the reviewer declined to review (e.g. "I'm too busy but at first glance this manuscript looks like a simple extension to a known experiment leading to unsurprising results, so is not a good fit for your journal") or there might be a confidential review that the editor isn't sharing.

If you really want to publish in the journal, you could write back to the editor saying you're willing to wait while they find reviewers, but otherwise yeah, submit to another journal.

This is a little unusual, and it's hard to know what happened. I have seen many papers accepted on the basis of a single report, even if the editor solicited more. One possibility is that the editor contacted potential reviewers who declined to provide a full review, but did make negative comments. It is also possible that the editor has some concerns about the manuscript, and felt that the one positive report did not adequately address those concerns. Finally, the editor may have felt that the number of unwilling reviewers was unusually large (if the paper is so great, how come nobody wants to review it?).

The whole thing is obviously somewhat unfair -- the whole point of the referee process is to provide useful feedback, and this did not happen in the present case. Most reputable journals have an appeals process. While you may not be successful in getting the paper published, you may at least get more useful feedback.