Here's the thing: editors have developed a habit of hiding their heads in the sand. I can speak on behalf of not all editors, but most. We have so much work--so many deadlines to meet in-house, and so many proposals and manuscripts to read--that if an agent doesn't follow up about a manuscript there is a 9/10 chance we're not going to read it (certainly not going to buy it).
When an agent doesn't follow up, they're demonstrating a number of things, the foremost being that they don't EXPECT the book to sell. "Oh nope, this is no hot property, take all the time you need" is the message we get. And honestly, it takes 15 minutes for an editor to know whether they want to read more or not--not 4 months, or 6 months. Sure, we need a kick in the pants to pick the manuscript up. But if you wait 4 or 6 months to give us that kick in the pants, we'll never, ever, ever get herded into, say, auction on that book.
Read the rest.