Costs and a declining appreciation for quality. It is no secret that Winchester has struggled to make money since the early 60s. It has changed hands, closed factories, and discontinued guns. I'm not sure they are making any guns any more. The iconic guns that made Winchester (the M-12, 70, and 94) were also more expensive to produce than the stamped guns Remington and others made. Winchester has simply made cheaper guns, particularly pumps, at every opportunity since then.
The second factor is that fewer people today care. Just look around. 100 or even 50 years ago a gun was a piece of your life. Something you used to feed or protect your family. American made quality was expected. Today people just want something cheap that will go bang. The overwhelming majority of "hunters" wont shoot enough in an entire lifetime to appreciate the difference in quality between an 870 and a 1300. It's a simple and sad sign of the times. Call it the Wal-Mart effect, or the increasing urbanization of America, or whatever.