I have not been given this exact feedback, but I have received my fair share of desk rejects. I am also a new editor myself, so I have a bit of insight here.
There are usually a few reasons why an editor might reject a paper prior to review.
- The paper is not within the scope of the journal.
- The paper is not impactful enough for the journal.
- The paper may be decent but is lacking in one area or another and the editor just isn't interested i.e., they have better submissions.
- The paper is just generally below the quality generally expected at the journal.
- The paper is so poorly written or so obviously flawed that it does not need to be sent for expert review to spot the issues
There is nothing to do about 1, other than be more diligent when picking journals to submit to. 2 is also difficult to directly address and is somewhat subjective, usually the solution is to just move on to the next journal. Option 5 is unusual for a decent academic. It takes laziness or a lack of experience/knowledge to churn out a legitimately bad paper. Think of the type of paper that ends up in a predatory journal.
Options 3 and 4 are probably where most desk rejections fall. These are also the two possibilities that really leave room for improvement.
Often you will receive no feedback from the editor. In your case, you have a clue. They told you the paper was "not developed enough" - that probably puts you in those middle categories (3 or 4). I would take that literally.
So what do you do? Well the best thing you can do is try to fix the "problem". A paper being undeveloped could mean a lot of things. It could be that you introduced some interesting ideas but you never expanded on them. Or it could be that you have a strong foundation but never really introduce anything new, even though there was the potential to. It could be that the paper needs editing for structure, clarity, flow, etc. It can be hard to identify these issues yourself, so you should find a fresh set of eyes (preferably more experienced and knowledgeable than yourself) to help you revise the paper.
A final thought, I would not recommend resubmitting anywhere without at least trying to revise the paper and "develop" it more. It could be that the editor was just not interested and made up an excuse but considering that editors generally don't need to give you a reason for rejection, you should treat their feedback as true for now.