Geekhack, it seems to me, is of an earlier time when mechanical keyboards were less well-known, and more of a 'niche' hobby.
You had people getting excited about SP or GMK producing basic sets, and many of the first real good GBs were set up here. GH created a number of things that are taken for granted now, like Moogle kits and so on.
But it's from a time when Cherry switches weren't necessarily considered 'standard,' as many of the keyboard users here had no intention to move away from buckling springs, Topre, Matias, or more obscure keyboards that are no longer made. 'Mechanical Keyboard' on GeekHack meant just that: a mechanical that is just as likely a Model F as a Cherry MX Red.
But mechanical keyboards took off in a big way in the last 5 or so years, and for the huge influx of new users, "mechanical keyboard" means "RGB MX Gaming Keyboard." The explosion of MX and clones enlarged the subgroup of actual Cherry enthusiasts, which is a large part of what the Reddit group represents.
So the new wave is Cherry-inspired, it derives from the popularity of MX and the burgeoning clones. A number of them own BS keyboards for cred, and the hardcore know a lot about the classic designs, but the hobby is really now about customizing Cherry.
So GH belongs to an earlier era, before mechanical keyboards reached mass-market acceptance, when people were still excited they could get Dolch and Olivetti, or modernization keycaps for their old sets.
As someone who became involved in all this a year-ago, it was clear to me that 90% of the action was on Reddit. I still found GH links because they come up when you search for things, but I think most people are going to Reddit to ask their questions. The bulletin-board format is considered outdated compared to Reddit, and now Discord, so it increases the 'niche' aspect of GH.
GH is kind of like a storied old library that contains many interesting texts somewhat forgotten, where people still go to get access to certain materials that aren't found elsewhere. And there's a group of dedicated volunteers (group buys) performing a community service that brings people to the library. And Reddit is a giant bazaar where everyone is going, where people proudly display their keyboards and stuff.
I think it's kind of sad that keyboard discussion is divided up among various websites, but they represent different times at which different groups of people coalesced. And the Reddit group does seem like a place where people mostly post pictures of their keyboards instead of engaging in finer discussion. The discussion of specifics has moved to Discord, where it's become little societies. So that's where it's at right now.