Why do we struggle with depth in some positions but not others?

After watching last night's disappointing 2nd half against France, I've been wondering why the wallabies seem to consistently be able to develop talent in some positions but not others.

For example, number 7. I grew up with David Wilson, then Phil Waugh and George Smith, Pocock and Hooper and now the man-who-we-need-5-clones-of, Fraser McReight.

We seem to have always had plenty of quality wingers and fullbacks, and now that Aus rugby is in dire straits, still do (where is Lomax gonna slot in?).

But yet, one or two Fly Halves get injured and we're struggling. Same with scrum half, we've thrown Tate back into the mix who clearly isn't ready or back to his old form.

Using a cricket analogy, we've historically produced great fast bowlers because our pitches are bouncy, and that's due to climate. Conversely, we don't produce as many spinners as say the subcontinent because pitches don't turn due to our weather.

I'm wondering what it is about Australian rugby conditions that means we have a plethora of depth in some positions, but not others?