It is a quite high threshold, in learning effort and time, to contribute. This could be one of the reasons that the contributions might be "privatized".
Well, it's human nature. If I write a new shader for, say, the Shuttle, I can just leave it there - the effect I want works, end of story, I got what I wanted for my effort.
Or I can put it to the data repository. Then I actually need to re-factor it, make it configurable and fit more use cases than my own, need to integrate it with the rest of the framework, need to take care that configurations I don't use still run, document it and provide support for it. And read through forum complaints that things are poorly documented.
The incentive to put such a thing to FGData is just not existent, because it works for me. The only reason to still do it is that if everyone else also does his part for the greater good we're better off than if everyone does things for himself.
It's basic game theory, the
prisoner's dilemma. No matter what the others do, you're always better off if you just do your own stuff and don't care for the common good. Yet if everyone follows that maxim, we all lose, whereas if everyone does something for the common good, we all win.
If you calculate a reasonable timeframe to learn something, it can usually be done. It took me a year to write good GLSL shaders, but it was a year of hands-on experience - no documentation can replace that. It took me two hours to learn to use the FG particle system. A month to figure out how canvas really works. A few days to learn how to use JSBSim for the flight dynamics (yeah, that was love on first sight....).
If you expect to churn out new scenery after a day, you'll be disappointed - but I believe if you take two weeks to learn it, you'll get somewhere with the tutorials provided. And maybe after a year you'll be quite good at it.