Originally Posted by Loomdun
Its just as close to the dragon and allows the supporter to ward both top and bottom and dragon without being heavily at risk of gank. Whats the deal with bot
To answer the OP's question.
Like others have said, the side lanes are harder to gank than the mid lane. Therefore, you put the character with the best escapes there. Also as others have said, whether you are purple or blue, you would like to have the character that benefits most from blue buff be in the mid lane so that whether you want to take theirs or yours, that character is close by. And so, the AP based character will go solo mid because...
a) they generally fit the requirements outlined above (those that don't aren't seen very often)
b) they work better off of experience than off of gold (as compared to an AD carry) and so they can afford to ward themselves and not have a babysitter do it for them
Hope that helps!