It's a marketing decision. There are more 870's out there than there are Mossberg this is evident in how many more accessory options are available to the 870 over the Mossberg's.
Although, because of so many adapters out there, most 870 stocks will fit to the Mossy's. Which again brilliant marketing on Magpul. "Oh hey! You got a Mossy? Great, with a simple 870 adapter you can have this unfair advantage too! Unless you don't mind waiting a year to get the specific accessory!" In most cases people will buy the 870 part and use the adapt, but in many cases they'll do that and then the right one later on if they liked the stock. Some might even buy an 870 to put the first set of furniture on ((since 870's can be dirt cheap used)). Either way Magpul makes bucks.