Have you not noticed that Wiki is having real problems with vandalism and spam and is rapidly making only registered users able to edit there?
Sadly we are very vulnerable to spam and if we were not very careful with selection of editors then the whole project would have collapsed years ago.
I am sorry that you were not accepted, many editors find they have to apply more than once to be accepted. I see no new editor applications because I am not senior enough to deal with them, so I can only offer advice from my own applications and what I have read. But often rejection is because one chooses too large a category. Between about 30 and 80 listed sites in that category and in any sub categories in TOTAL. Also if one picks a category susceptible to spam, then one might be rejected. However highly skilled in the area that the category represents new editors are just that and have no experience in editing. The advice to apply for a regional category was good advice. It is based on the idea that you can learn the basics on developing a category where you certainly have some local and invaluable knowledge because you live there. But that one category is not the end of the story, if you show that you grasp the elements of editing, and you probably would quickly (some really don't) then one can apply for larger and more complex categories. I don't know what you applied for, but I guess from what you say that it was fairly high up a tree and had many sites in it and its sub categories. An editor can edit anywhere down the tree from where they are listed as editors.
The other main issues are be honest about any sites one is affiliated with, and also about reasons for wanting to edit, applications are monitored to ensure that the applicant is not JUST trying to become an editor to list their own sites, though listing ones own site and competitors sites is permitted.
Then ensure that the three sites suggested really do fit the category and are unique content. Show you have read the guidelines with the title and description, we don't want people who ignore what the community want done. They are also looking to make sure that the applicant has a basic grasp of grammar.
Many of these will, likely, not apply to you I added them so that others might see some tips if they want to apply.
But please apply again, if they really had not liked your application they would have said don't contact us, we will contact you if there ever is another vacancy. Yes, we do tell people not to bother applying again!
Whilst we need editors and whilst we need them with knowledge and the sort of esperience you seem to have, we also have to ensure that we have criteria to ensure that we get the right type of editor with those skills. The application form is a sort of test as we have to try and protect the directory. That may mean that we miss some people who would be brilliant, but hopefully it also weeds out the people who would be disasters.
Hope that helps
My opinions are my own and do not represent DMOZ, staff or any other editor.