- UX surely played a part in it, especially for the oldest of forums that collapsed the thread by default. Then again, Dpreview still has that layout, and while it clearly wasn't successful enough for Amazon to want to keep it on, it still has a loyal following.
- Hiding content—or rather, only showing low-quality image previews to logged-out users—makes the "drive-by" experience less nice. HN—and Reddit, to a lesser extent—avoided that enough by not allowing images in comments, leading to users creating their own workarounds.
- Profit, or the lack thereof, likely was a larger problem. Banner ads just don't make enough profit, especially compared to Facebook et al's in-feed ads.
- Maybe the collective internet just isn't content with the smaller crowds that would have seemed great in the forum heyday that, today, pale in contrast to the traffic from one viral post on social media.