I think it boils down to something pretty simple.
3.5 - Tons of tactically interesting choices at every level. Depending on what you play, you can also get choices every time you play a session. (What spells do I prep, which monster do I summon)
4e - Way less choices. All of them are uninteresting. +1 to hit, 2W Damage and slide 1, or 3w damage and push 1. You only get choices when you level.
With online boards, you have people constantly theorycrafting builds and running through various combos and very quickly the "Correct" 4e builds were flushed out. Conversation didn't immediately end because WoTC would throw out some errata that changed stuff, and that might revive it for a few weeks. Optimization basically became "Play one of the top tier builds, which are all some variant of ____". If your group doesn't optimize much it was "Play a tier 2 build, A, B, or C is good". If you don't want to optimize, why would you go online to discuss your build? Go somewhere edition neutral and talk about his story or plot elements.
4e isn't fun to talk about, so no one does. I suspect once 5e is out, there will be a strong 3.5 community, a weak 2nd ed community and a respectable 5e community (but based on what I've seen, I don't see it being better than 3.X, and so I don't see much migration)