you might want to do some general googling on wireless networks / radios / bands and such. there is lots of information out there.
quick answers
1) because a radio "works" on one or the other, a in 5ghz and b/g in 2.4ghz
2) n can work on the a 5ghz or b/g 2.4ghz, it brings some tricks for higher throughput
3) one could say a as it is less used, has more usable channels, and can do better channel bonding; but not all devices support it and the signal reaches less far, so with most things, it depends.