Because bios are boring. Usually. Front Porch has some good ones. "Bacon." "Catfriend." "SuiGeneris." These are some good words. I don't want any part of the journal to be boring.
Also, I worried that I was being overly influenced by bios. Like, if a bio is three pages long, and they list all 300 online magazines they were ever in - then I found I was not paying very close attention to their actual work. I just immediately hated them. There are a lot of opinions about bio etiquette out there. I guess I'm in the short-and-sweet camp. And you can't get any shorter or sweeter than savoring the delicious, etymologically rich depths of a single word.
Many times the best writers have the best one-word bios. But sometimes not. Sometimes their bios are dumb. They still win because they are the best writers. That's what matters.
Reaction to this change in our guidelines has been varied. Some people get mad. This is the wrong reaction. It's supposed to be fun. Some people say things like "writer" or "poet." Don't do that either. I already know that about you. Teach me something new. Some people just ignore it and send a regular, 3-page long bio. Maybe they think I'm joking. I'm not. I take your word choice very seriously. It's supposed to be fun. But also it's very, very serious.