Email flows fucking suck ass. Yes I have used them. No, I won't budge on this, and no, I'm not going to go proselytize on LKML or Sourcehut or whatever about it, in Rome I'll just do as the Romans even if I think it sucks. But I've used every strategy you can think of to submit patches, and I can't really blame anyone for not wading through 500 gallons of horrendous bullshit known as the mailing list experience in order to glean the important things from it (like range-diff), even if I'm willing to do it because I have high pain tolerance or am a hired gun for someone's project.
Also, to be fair, Gerrit was released in 2009, and as the creator of ReviewBoard (in this thread!) also noted it supports interdiffs, and supported them for multiple version control backends, was released in 2006! This was not a totally foreign concept, it's just that for reasons, GitHub won, and the defaults chosen by the most popular software forge in history tend to have downstream cultural consequences, both good and bad, as you note.