Editors are traditionally myopic, and work in offices cluttered with manuscripts. There are meetings and book fairs and sales conferences to attend, authors to entertain, and serious editing to be done. There is a lot of pressure, and time is in short supply.
Editors read manuscripts from their favourite agents first.
Then they read manuscripts from other agents. Last, they wearily attack what is known as 'the slush pile' . This stack of unsolicited manuscripts is, presumably, so called because, like unwanted snow, it piles up and turns dirty and yellow with age. Some publishing houses will not even allow a slush pile to accumulate - they simply return unsolicited manuscripts unopened. It's human nature for an editor facing a large slush pile to read short submissions first.