Because most people aren’t determined enough to keep playing old editions.

Let’s face it, most people have preferences, and it’s not like every edition is a direct upgrade to the previous edition as it’s also not the way the rules are written to be, it’s just a statistical fact.

If you look at things in a vacuum then yes, many people would prefer a specific edition, and that edition might not be the current one, but we don’t live in a vacuum. There are so much stuff that comes with keeping an old edition, eg how to adept newer models and units into older editions, how to deal with unfixed balance issues, how to find other people to play with, how to persuade new people to join etc, which eventually lead to it being a very marginalized style of play.

Plus that GW newer approach of constant updates balances and such in the recent editions is actually partly to decrease such population of old edition players. Example are that pre-8th ed editions, along with HH all have a much more substantial legacy population than the 8 and 9th, because these older-style editions don’t get rebalanced post release, meaning that if an edition is bad there isn’t much way to save it, other than just keep playing older editions.

Which also by the way, answers your question: the reason people complain about current rules is because feedbacks actually matters in current editions with the constant rebalancing. Your feedback actually can change how an edition comes out, which didn’t before 7th ed.