User blog comment:TheTobarMethod/ADC Mid....It can work!/@comment-137.226.66.4-20130129163124

As someone else already said, adc mid used to be a common thing. At least I saw a lot of them back when I joined which was... err.. 1.5 yrs ago or something?

Anyways, the main reason you don't do it is probably the ganking. Most mages can easily push mid to disappear top or bot for a burst-dmg gank, while few adc really excel at pushing at bursting. Many mages also usually have stun abilities to help with the ganking (or defense, when caught between lanes) or some high mobility mechanisms (e.g. Ahri, TF) that most adc lack and which can be used to quickly change lanes and again for escape.

Of course, there are exceptions to the rule. Someone mentioned Sivir, who has a lot of utility and can almost be called an ad caster, so she works great mid. However, her ganks are probably kinda meh.

TL;DR: most ADCs lack the utility to be a really good pick mid.