Mostly because many mages are quite valuable for teamfights but are pretty vulnerable on the long sidelanes, so putting them mid helps to get them save into midgame when they really show there power.
Back in the days, adc also were alot weaker early on whil mages had a strong early-midgame teamfighting, so you would want the mage for those objective fights while the adc often tayed in a sidelane to farm up. Assasins on the other hand had strong roming duering lane, so haveing them mid to acces both sidelanes is benefical aswell.
Additionaly, it's much harder to punish a 1vs2 on the short lane.
If you have a dou mid and the opponent has it bot, their sololaner will have less trouble than yours.
However, with adc beeing relevant much earlier in the game and toplane haveing a relevant early onjective aswell, moveing the duolane mid has certainly some merit for early objective controll, though you need to be carefull with your sololane pick, the long sidelane is a dangerous place for many champs while most midlaners can defend their tower 1vs2 rather well.
*Edit: Forgot about the 'we don't want to enforce a meta but do it anyway' changes against laneswaps sin S6, so For the first 5 min, top and mid tower only take half the damage. Combined with tower-FB, this means not haveing your dou bot can cost you alot.