How to Have the Best Game of Warhammer 40K Ever 

A few thoughts to consider on why people still play older versions of DnD, but not 40k. This also pairs into why 40k continues to skew into meta-chasing. 1. DnD is co-op, 40k even in the most casual sense is competitive. For a co-op game you only need 1 person with the rules and willingness to run it and you're set. For a competitive game you need to find a group of people all with their own armies and rules, which is made harder if you're not playing the same thing. So naturally play gravitates to the latest version of what's most popular. 2. 40k is not backwards compatible, but it is usually forwards compatible. My primaris Salamander army can't play against my brother's 3rd edition Ultramarines marines since no datasheets exist for primaris. He however we can play in 9th or 10th using legends rules for his units just fine.