I also got rejected in the last few days. I was sent this link from my nomination application rejection, which may provide some info. I'd take a guess that without having a specific niche, Deep is at a disadvantage to get it's editors selected.

Additionally, you really need to have some type of expertise that makes you qualified for the niche. Photography pub editors should be photographers, coder pub editors should be coders, etc. Maybe with personal stories, editors should have something in your background how that gives you the expertise to identify stories.

https://blog.medium.com/bnp-editors-9c0a6f5a133a