I really don't think that is true though. Take a look at Baldur's Gate 3 for instance, it launched with numerous classes all of which are equally supported by the game, yet Paladin is the most picked class by a fair margin. For races half elf, human, and elf are the most picked with dwarves for instance being low picks. I've seen this sort of ranking in basically every popularity chart for a fantasy game , lots of people like the power fantasy of being a big armored guy and not many people like to be a dwarf. So even if GW put a lot of effort into Fyreslayers I don't think they would be nearly as popular as SCE. I think Marines are the #1 faction in 40k because they have the clearest and most consistent power fantasy that appeals to a lot of people while allowing a high degeee of customization, not because GW supports them them the most, they support them the most because people want it.