I am sure there are various reasons. I have an 1100 Remmington, and two 870 pumps, the tube extenders are on the 870s the main reason, ease of loading, to load an 870 you push the round in, with the 1100, contending with the button to release the door is a pain. The other factor is known reliability, over 30 years I have fired thousands of rounds through my 870, not once has it failed to function properly.
I prefer # 6 or # 7 1/2 shot for home defense, it will probably penetrate my exterior wall, but not both mine, and my neighbor's.