Besides Crucial/Micron, it seems like consumers can't buy DIMMs from Samsung or Hynix; you have to get them from some gamer brand. And clearly a lot of people are buying from brands that they recognize.
What makes you say this? I always buy Samsung DIMMs. They are always in stock at Newegg and on the shelf at Central Computer (SF Bay Area brick and mortar computer store).
Maybe I was only looking at 3200 or faster.
You can buy DIMMs from all three just rare to see in most retail channels for consumer stuff. Obviously Micron have crucial, but you see Micron non-server DIMMs in OEM systems.
memory.net has great lineups and cheap for Samsung memory.