I agree that there seems to be no rhyme nor reason (have plenty of mates who have failed at both stages) to the process but there are plenty of ex-skippers from a wide variety of backgrounds in the company (me included).
I'm just glad there will soon be FOs both older (just!) and junior to me!!!
But don't be phased by all those 40-something cadets with 25 years to do at the top of the tree blocking all those coveted command slots. I'm 40 with 3 years in the company and I doubt I'll see a LH command.
Good luck one and all - I still think it's the best place to be.