Because they are (typically) inexpensive, work every time, and don't shine on magazine covers with the same luster as an AR. Plastering 4-5 jpegs of your plain-jane 299.00 870 Express doesn't have the same impact as pics of a 1500.00 AR bearing a 600.00 optic.
There aren't thousands of threads on how to maintain them because they're not so finicky.
They're usually not in the politicians' crosshairs. They 'only' hold 5-8 in the magazine. Heck, even during the worst of the ammo drought, you could always find something on the shelf in 12/20/.410 gauges. So the stories just aren't there.
That's why I love mine so much. Short stock, sidesaddle, forend light, and sling--put those on when I got it 14-15 years ago and haven't felt the need to revisit it yet. Just keeps on going.