Top: The most convenient lane to gank (Mid is a fairly short lane and sometimes hard to gank, bot is warded most of the time by the support so he can farm safely) because it is fairly long, Top also (usually) has the tankiest people up there, so if they get ganked, they can withstand it slightly easier. Other then that, bruisers are level dependant and farm dependant so they can be tanky enough to go into the enemy team and soak enough damage.
Mid: Level dependant, is in the "middle" of the map, so no matter where stuff happens, the AP carry can react and help out accordingly (both rivers for example, dragon, both buffs and technically baron if people would be getting it that early which never happens) They also have alot of damage and since they are higher level than anybody else in the game outside of top, they have an advantage over anybody. Also note that in the laning phase, the most action happens at the bottom river and dragon, which means Top is never even present unless they have teleport, which means the AP carry will be higher than anybody else at the skirmish. Also the closest to blue buff on both sides.
Bot: I think the meta started to put AD bot when people started using AD carries started using wriggles, which makes you get Dragon really fast. AD Carries also are incredibly farm dependant, so if they get shoved out of the lane, they will get outfarmed and ultimately fall off mid and late game. This is why they get a support, they help the AD carry to live through out the lane, they ward key objectives and other stuff if needed so the AD carry can be safe. If the enemy iniates on your AD carry, your support may help you to keep them off of your carry with heals (Soraka) CC(Leona/Alistar) or damage. Sometimes they're even mixed and may have all 3 of the above mentioned things
Jungler: Self explanatory
Also, saying stuff like Sivir mid could be good wouldn't shift the meta. It is called a counter a pick, generally it still isn't done because an AD carry with level advantage isn't that much different, while an AP carry is ALOT different.
Putting bot with Soraka works, but it limits you to getting Soraka, which makes it prone to counter picking. Also another factor is that you need an ap carry, and usually mid is the only one with AP damage (If you aren't running double Wota stuff or have AP carries like Fiddles in the jungle) so if you have an AP carry mid and bot, you'd lack an AD carry. You can't put the AD carry top either because he will get owned by the bruiser, he can't go mid either because he'll get owned just as much.