Personally I think it's just a better marketing.
There were projects, sure - but they rarely sold in large volumes. That meant not many paid developers. And if you had a problem you'd be lucky if someone online had even experienced the same problem, let alone found a fix for it.
Every chip vendor seemed to have their own kernel fork with their own patches, and didn't much care to get them upstreamed. And of course, their kernel would be ancient - they're selling it for things like automotive infotainment systems which don't give a shit about security.
Want to attach, say, a battery-backed RTC to your SBC? I hope you know the difference between a dtc and a dtbi. And even if someone else has been nice enough to get the same part working on their SBC, just because a TI processor needs the line 'dmas = < 0x21 0x1c 0x00 0x21 0x1d 0x00 >;' doesn't mean you can copy that to a Freescale processor and expect it to work.
More than once I brought SBCs that came with a connector for a MIPI camera - but no camera available, and no software either. You want a camera? Plug in a USB webcam.
And let's be honest: If you want features like wifi and hardware h264 support you're going to be dealing with wonky binary firmware anyway.
And then the fact that you can still run a supported modern OS on an almost 10 year old board... that doesn't seem to happen with other SBCs (most are basically shelved after a couple years), and you end up running ancient OSes on them or obscure community-driven OSes that break in a hundred different ways.