its the one and only spot you can put an AP caster and get them a gold lead safely.
you can put a veigar top, until hes 0-11 and you instantly regret that decision because he just gets tower dove repeatedly.
its not that mid lane is over popular, its just the safest lane to be in and a lot of champs need to be played safely or dont have a lot of room to play aggressively.
basically mid is popular because you can put the most champions there and make it work where other lanes you can put yourself at a huge disadvantage for your champ choice. so all the people who want to play those champions, have to go mid, artificially creating a large mid lane population.