To break it down there is a general consensus that the 3 hardest roles in LoL go either Jungler > ADC > Support or Jungler > Support > ADC. There are some other fairly well supported role orderings but those two are the most common from my experience.
These 3 are broken into 2 types of difficulty. Mechanical difficulty and map awareness/pressure/control difficulty.
ADC's are the most mechanically difficult champions. Some people argue that they aren't because right click hur durr however there are rarely any champs as targeted and with as few tools to defend themselves as ADCs. (Which is Why ADC's get the support and not the mage laners) This is why they score high on the list at 2nd to 3rd place. Mid or Top as a role are not as difficult. Sure there are champs like Azir that are hard and on a comparable difficulty to the hardest ADC's but as an overall role your mechanics don't need to be as high.
Junglers and Supports require the most map awareness. They are the ones who roam, gank, set up vision, often call the shots, etc. etc. That takes a lot of game knowledge rather than mechanical skill. Junglers tend to take more as they decide the game early on while supports just want to stick with their ADC. Mid - Late game though their roles are quite similar and it is their job to engage/peel/decide when and where to fight. They need to know what fights can be taken, how much room for error there is, who the priority lockdown, damage, and peel targets are, how healthy the team needs to be, where to set up and clear vision etc. etc.
Other roles help in this but their job is often to provide the damage/tank not to engage, ward, clear, call off fights etc.
That is why support and jungler are placed so high, they need much more game knowledge and map awareness than any of the other roles because that is their job.