1.) why is there an 'ap carry' mid?
mostly because the mage is an early source of damage that can help control the other lanes with ganks. also because of mid lanes proximity to the blue buff. lastly to help control dragon.
2.) why adc + support bot?
to control dragon.
3.) why bruiser top?
cause there is no other lane open to put a tank in to farm.
4.) why jungle?
cause babbys need a gank to help win a lane (or to avoid losing it).
5.) why is there no other viable meta games being played?
cause people are afraid to try something new when the current meta works fine for most players.
1) Why is there an AP Carry mid?
AP Carries tend to farm with their abilities once the mid game is reached. This, plus a feed on blue buff, allows them to push waves and roam with the jungler. They can clear farm quickly, and therefore lend to map presence. The reason there is an AP carry at all makes it more difficult for players to simply "Build armor and win." Their combination of utility and damage output forces players to make decisions on itemization beyond "everyone buy a thornmail and win the late game". Furthermore, AP Carries tend to have some form of mobility/cc built in (with certain exceptions of course) thereby, despite their squishyness, allowing them to sit in mid by themselves because it is the shortest lane from outer turret to outer turret. When I jungle I frequently call upon the mid player to help me counter-jungle buffs.
2) Why is there a support+adc combination bot.
The ADC is less flexible and cannot farm with abilities such as the AP Mid, therefore it is somewhat impractical to run them in the middle position because they would not be able to take advantage of the central position to increase their map presence. Therefore, they are in an extended lane. Furthermore, most ADC have less utility/cc/escapes built in than AP Carries, so putting a support in a lane with them gives them that utility/cc/escape that covers them from ganks and allows them to disengage from a bad fight. Finally, as to top versus bot, while the recent finals have shown a fair bit of top play from the adc/support, the primarily reason I believe is simply Dragon control.
3) Why Bruiser/Tank top?
The Bruiser is effectively a mid-game position. This champion is there to ensure the team has a little bit of everything needed to win midgame fights before the ADC/APC damage is dominant. You find that bruisers typically underperform in the late game unless they have some form of strong initiate and built more towards tankiness or a strong gapclose and stickiness to kill carries coupled with more damage. The reason you play a Bruiser or Tank top is that they will have a bit of utility, and enough hp and armor to make them survivable on their own in a longer lane.
4) Why have a jungle?
There's a few reasons for this. If you ran a double top you'd be crippling yourself because your top would receive less experience and if both are farming, gold, than if you had a solotop. Furthermore, running a tank/supp top means there is no one to control map objectives and feed buffs to the lanes. There is also the fact that you would effectively be not taking advantage of a great deal of money/experience that is sitting on the map unfarmed by your team. Finally, the biggest reason is map control. A good jungler will be carrying oracles and be warding, thereby denying the security that wards provide to lanes, and helping snowball the carries early with protective counter-ganks and ganks on lanes that are easier to kill/drive out of lane. The jungle is the primary source of map presence in the laning phase. It is also probably the most misunderstood and poorly executed role in low-elo gaming.
5) Why are there no other viable meta games being played?
It's hard to see a meta that would be more successful than the current one for the reasons I just listed. I understand running adc/support top if your nexus is on the upper right side as that lends proximity to red buff, but beyond that there are few advantages towards shifting this meta from my vantage point.
tl;dr Well ****ing read it because its too much to summarize.