Why we don't have more junglers and supports
Aside from them being the positions you have to know the game and make calls from, I think I know why more players aren't supports or junglers.
Nowhere to practice and learn, particularly early in their LOL experience.
When you are just starting out LOL and after you've done the tutorials, it's on to games. There's basically 4 options.
- Aram - no true support (laning wise) or any jungle
- Bot games - no bot junglers to worry about (from what I've heard)
- Custom bot games - I'll get to this
- Normal games - have to deal with real players while learning two difficult positions.
Custom bot games are a bit of a problem and require more than 1 line to talk about. Frankly they suck for a real game experience. They are good for learning the basics to things like CSing and figuring out combos, but that's it. If you are trying to learn to be a proper support, this is just an exercise in frustration. For a jungler, you get no leash, you have no runes, and you never get to learn about counter jungling or being counter jungled. You also have to camp top which in real games isn't going to happen all that often.
For both roles however, there's a real killer: you don't learn when to make calls for dragon or baron. Those are key for those roles and the AI doesn't listen. You may or may not have a smurf on your team early on who can teach you those things either.
For beginners wanting to learn the jungle, as I said you've got no leash, no runes (need armor), and no masteries. Some champs can clear blue/red buff (nothing to teach you about jungle clearing order either) without a leash. But without runes or masteries either? Maybe someone who's good with the right jungle champ maybe, but for a beginner that seems like a tall order?
And for supports... being a support is rough enough as it is. Now add in a limited knowledge of the game and being stuck with teammates that also might not know how to play the game.
So I think a big reason we don't have more players who will fill those roles is because there's not a good place to learn those roles short of doing a lot of homework and then being thrown into the fire.