The publishing world has changed from the days when writers would type up their manuscripts, slip them into large manila envelopes, complete with a SASE (self-addressed stamped envelope) for the response from the editor or agent, and mail it to the agency or publisher of your choice.
Now everything is done electronically and some writers flood multiple editors with copies of the same manuscript and they felt so overwhelmed that they decided to close, to not accept submissions anymore, except maybe from agents. And the few who accepted submissions decided that, if you have't heard back in X amount of time, it's a rejection. (leaving the writer to wonder if they even received it)
Even multi-published authors have been trapped by this closed door policy.
Laura Backes, editor and publisher of the Children's Book Insider newsletter and now involved with WritingBlueprints.com, has written a letter to editors and agents discussing this problem.
An Open Letter to Editors & Agents, It's Time to talk about Rejections.
Click on the link. It's good.