The fluff states that not all Marines have taken the Rubicon Primaris. Further, that not all Marine chapters use Primaris.
Your argument extends to the claim that Craftworlders have been soft-squatted because Marines get more attention.
If your a new player like me. And just go through the lore it. It is a battle brother becomes a venguard , then runs around with a hellblaster, etc yeah bikes and land speeders are mentioned, but if you don't know in advance that those can't be used by primaris or aren't maned by them. And then GW forces primaris even on chapters that should really not be willing to accept them at all. And right now even DA have them in their inner circle. Hey even the FT took them in. When was the last time we got a story about normal marines? maybe it was in the black library books, and even there am not sure if it was true. Because the two I read about the new 8th ed lore were super strange, with mutant primaris chapter masters and classic marines killing themselfs including their gene seed because they were genestealer agents all along, and only the new primaris are the real cool marines.
If you're a Marine player, your codex probably doesn't discuss Necrons very much. That doesn't mean Necrons have been soft-squatted.