The Remington Model 870 is simply the most successful shotgun ever made, having sold more units than any other shotgun in history. It was originally designed in 1961 and was essentially a more inexpensive development of the earlier Model 31, which was extremely well made but also extraordinarily expensive and out of the price range for most folks.
With the 870, Remington sought to make a shotgun that was very durable but that could also be offered at a small price. Remington was able to reduce the price even further when they introduced the 870 Express line in the 1980s. The Express was cheaper due to the black matte finish.