League of Legends is a highly solved game, that is, it is already understood how everything should be done optimally.
Riot Games Mob is designed to be played in a certain way, so that today we always have a player on the top route, one in the jungle, one in the middle and two on the lower route.
This was not necessarily the case at the beginning of the game and this distribution became mandatory in part thanks to the professional scenario, which greatly influenced LOL development over the years.
But how did this happen?

• A default choice?
• When ADC’s and Supports invaded the top

A default choice?

During Beta and season 1 LOL it was common for the ADC’s to occupy the middle route.
If you look at community creations at that time, you will realize that seeing snipers in the middle was a norm.
After all, they are fragile champions, very vulnerable to banks, so it made sense to put them in the shortest lane of the game, which allowed them to score with relative security.
But the most involved players and professionals quickly understood that an ADC needed more items than experience.
On the other hand, wizard champions that depend more on skills-or be, on levels, occupied the side routes.
If shooters needed gold to buy items, but they are still fragile and vulnerable, it makes sense to put them with a pair that helps you survive and do not need gold or much experience.
Of course the idea of placing wizards in the middle and the ADC son the sides was emerging, but still why on the lower route?
In the past there was no herald of the valley and Baron Nash or only appears in 20 minutes, so it would not make sense to concentrate this pair on the top route, especially considering that the dragon was on the lower route, where it is today.

In the past the dragon was a much less exciting goal, though impactful: granted gold to the team that shakes it.
Then it made perfect sense to put the ADC and the support on the lower route to create a numerical advantage situation to play the dragon.

Before the first international competitions, it was the European teams of the time such as AAA (Against All Authority) and Fanatic who popularized the strategy of taking ADC’s and ackets in the bot, as well as a wizard in MID and another on top.
The other teams quickly understood the reasons and began to join, from which it became the norm even in casual and ranked matches.

When ADC’s and Supports invaded the top

There was a time, in mid-2016, when, in the competitive scenario, it was normal that ADC and support were at the top against the to planer enemy.
The period is considered as an Age of Darkness in the eSports of LOL, as it promoted boring matches to watch, to the point where the whole team ended up sending the pair to the top, reversing the map as usual, but at the same time,
They changed all the time trying to force the situation of 2v1.
It came to the point where the designated to planer divided the Jungle with his own hunter, leaving the route with no one to defend.
It was common until sending four players to a route to overthrow the first tower without contestation.
To force the ADC and the supports to stay at Bot lane at 2V2, Riot Games made some significant changes in the game.
The different types of dragons arrived, as are the Elder Dragon, which gives the most powerful buff of the game in most situations.
Barricades were also thoughts designed for this, as well as several other changes involving the experience of troops.
Teams, therefore, have much more interest in playing in the bot side at the beginning of the game, to try to accumulate dragons, force barricades, etc.