Because the above 2 just focus on a limited range of parts for a limited range of cars.
IMO , having used both of them , they hold in stock a greater , and much more in-depth , range of parts for a much wider range of Mercedes-Benz cars , actually having manufactured reproductions of discontinued parts like body panels in many cases , for a much broader range of vehicles than the common motor factors who only do service parts plus a few 'faster moving' items .
The two specialists I mentioned do specialise only in Mercedes-Benz , unlike the factors who cover a broad range of European and 'pacific rim' vehicles , and could never compete in depth of parts stocked for each model ( try getting a complete floorpan for a W111 or W180 out of some of the factors )