I'm echoing other people I think, but one important thing is lane length. Generally, you want your solo player (The one without a Supp) to be the safest, since they have no help. Mid is the shortest lane, so a mid laner doesn't need to flee as far to reach a tower. If you put a duo in mid, you're doubling down on safety but putting your other solo laner at risk, and bot, in that case, would probably become home to more tanks or bruisers that can survive in an extended lane like top, and mages would drop off dramatically.
This will sound counter to what I just said, but bot is easier for two people to defend. Between an ADC and a Support with Sightstone, you can cover all of bots weak spots and ganking paths, but mid, there are a lot more; through the bushes, behind the bush walls, from the raptor camp, the little nook beside raptor camp (This was in worlds last night, raptors were warded so a jungler ganked from the little nook by raptors), and the jungle entrance behind the tower. It would be too much for even two people to keep track of.
And then there's Teleport. The ward placements in bot lane are very preferential to Teleport plays. Not only is bot easier to gank in this way than mid (Because of the lane length), but you're getting more efficiency out of it by traveling a further distance. That sounds like a reason why you wouldn't want two people there, but in this same scenario, it'd be easier for the top to TP to a bot with one person than to TP to a mid with two. Two people can defend themselves from a gank easier than one.
And this all goes back to lane length. Mid is shorter, so it fits a solo player inherently better than a longer lane because they can defend themselves without needing a Support to help them.
And then there's the very short distance from dragon, like you said in the OP.