If I may weigh in here... even though I'm the noobiest of noobs when it comes to the GBA... I've seen this with a lot of other retro forums. I've got quite some experience with retro computers of the '80s... try Sinclair QL, SAM Coupe, Spectrums, Amstrads, Acorn BBCs, Amigas, Ataris, the lots. Basically, there are two groups: one is the really popular machines that have thousands of users (like *eye-roll* the Amigas and the Commodores) and then there's the 'rare' division, with the most prominent example being the QL, where the situation is like you just described it: a few users making all the posts and then passer-bys (or is it passers-by?) that just come in, ask a question or two, some even make some grandiose statement about reviving the "community" single-handedly and then disappear as quickly as they came in.
Of course, I'm talking about computers and, by definition, their versatility is orders of magnitude bigger than a hand-held game console, as is their ease of programming. That's why we're still talking about a computer 30 years old, when the GBA is not even 6 years (from discontinuation) old and it's already getting into the 'retro' realm.
Having said all that, I can see some parallels between the GBA and the 'rare' retro division. There are few truly knowledgeable people around and even those have other things to do rather than keep developing for an old platform or keep answering the same old questions over and over again.
Even the youtube situation... when you've got loads and loads of """""reviewers""""" out there, some of them even kids with a webcam, one wonders if it's worth the time and effort to do the same thing -- even if the quality is light years ahead. The internet and its "freedom" also have downsides, one of which is... quantity often drowns out quality.
All in all... the GBA is a game machine; as such, the majority of its users will move on to greener pastures, sooner or later, after it's been discontinued. It's the same thing with the 'retro' computers, but even worse, since with games, the "latest and greatest" notion is even more prevalent. People want new 'stuff'... they get bored playing the same games over and over... they want newer games with better graphics and better sound and new features so, if there are no new releases, they'll just move on to newer platforms that have them.
For better or worse... that's how things work... :/