Has nothing to do with making money. What would be the standard? For a flush fit, or near flush, there is no standard. If you don't care if it hangs down a few inches, there's STANAG, AR10 and AICS if it's compatible with the cartridge you're chambering. There is no standard magazine for .30-06 length cartridges, certainly nothing bigger. A magazine has to be as popular and ubiquitous as those aforementioned or the 10/22 rotary for other manufacturers to adapt the design to their guns. So for anything else, they design their own.
I'm kinda torn on the concept. I would mostly prefer a slick bottom hinged floorplate or blind magazine on a boltgun.