Here's why. Say we get a query in the mail, in which the requisite 10 pages are not included (tsk tsk). Should we find your submission even the barest shade of interesting, we are less inclined to print out a request asking for those ten pages. However, if you send your query via email, it's just a quick reply.
Scenario 2: Perhaps I read an unsolicited query that piques my attention, but that we cannot take on for one reason or another. If the query had been emailed, I might write a brief note stating our thoughts, which I am happy to do from time to time. However, if it is unsolicited snail mail, the convenience of it all is dampened; it is unlikely that I will print out a personalized note to SASE back to the author, when the pile of such mail grows and grows like Act II of Little Shop of Horrors.
Lastly, it's environmentally friendly. And we here at Linn Prentis believe that somewhere, perhaps in the Puyo region of the Amazon, there is a brightly colored bird that has a brain like R.A. Lafferty, or is perhaps the incarnation of Lafferty, or is a distant-but-genius relation of Lafferty, and if we cut down his home, we will be without one of Linn's favorite Sci-fi authors. Again.
Good luck, fair Authors,