They are the Astartes faction's baseline rulebook and easiest to follow compared to other Chapters.



Lesser model needed to kickstart a simple game compared to Orks, Imperial Guard or Tyranids that required mountains of models.



Easy to paint models with there simple color schemes. Blues are cheap in Citadel.

To make the best impression of the pilot product they OP the Ultramarines and nerf the others, giving new players the first excitement of winning, enjoying the game and encourage them to buy more Ultramarines. When they are a seasoned player, they inevitably would want to try other factions.



JUST AS PLANNED

My Opinion:Space Marines are GW's best sellers. They are the pilot product to introduce new players to the game. That's where the Ultramarines comes in.