I don't think what you're asking can be responded to with a singular, concise answer. However, I'll poke at it a bit because I've seen other homebrewing sites rise and fall.
What exactly happened here?
Well, I'm not sure precisely what you mean. Homebrewing as a hobby seems to get blips of pop-culture which causes people to come in (and shortly exit thereafter) in droves. The definition "Homebrewing" is pretty loose, and it umbrellas competitive homebrewers, to fermented platic bags of sugar-water in someones closet. Also- a big factor is duplicates here simply are not closed. Questions here seem to follow (at least) a 50% duplicate rate. If we closed the duplicates -it would look much like the other sites in your picture.
Or the community started feeling discouraged from participating and people left?
Stack Exchange sites are intrinsically much different from places like reddit because there is some idea of formality to the question/answer format. For instance, most of reddit homebrewing is just pictures of bottles saying "my first batch!" with little meaningful content. That type of content isn't usually the norm for SO/SE sites. Here, things are question->answer. Elsewhere, things are designed for post->react. This means aside from answering simple questions- there isn't much of an interaction hook here (I'm not saying thats a bad thing)
And like other homebrewing (or ANY hobby) communities there's a particular growth point in which the community is 90% new folks asking basic questions, and 10% experts who don't really get any benefit from answering "Is it infected?" posts for the 1000th time, and they eventually wonder off. The experts don't really have much to learn from places like this because we've read the books done the work, there really isn't much continuing value for more experienced brewers aside from the fun of talking to, or helping other brewers.
At some point every homebrewing question had already been asked and there was nothing new to ask?
I'm going to go on a weird limb here and say, yes- every homebrewing question has already been asked. Not here of course- but in across the internet. The science of brewing seems to make interesting strides, but almost no advances have really changed much of how a typical homebrewer makes beer in the last 10 years or so.
From what I can tell, there's been basically little community policing here (not that it's needed typically) and what's you see here is probably very typical to any other specialty hobby community when left mostly alone.