its not about how "easy" it is
Is about gold efficiency. If you reach your power spike with just one item and 1 skill maxed. You don't need that much gold or that many levels to begin with. And that's what makes you ideal for support role, which, gets the lowest amount of gold and will most likely have the lowest level in the game.
I mean, sure, take the likes of Akali, she gets a power spike at
and
. But she still scales nicely with gold and wants more gold. And her levels matter a lot, at least till 16.
Or the likes of Darius, which power spikes at
. Even if its an early power spike, he still wants gold to ramp up in his kit.
However, champions with high utility that spike hard with just one item, hence why they are usually mages, end up in support role. As you can still use their utility and their powers even if you have to waste more gold than others on warding.
And usually, if you are not getting feed, the jungler is the 4th one in the team gold wise, above the support but ahead of the laners that farm minions.
Barring a few exceptions like shyvana or yi which can clear jungle very fast over and over again, this rule stays.
This is my simple view on it.
Same reason why malzahar went support. The entire power was shifted into W, so, just get a RY combo.
and you no longer care about your other item slots such as
etc, you can just build whatever as you don't need the AP to make it work.
Meanwhile, pre rework malzahar wanted RoA into RY combo into more AP as his abilities actually did damage if he got AP items.