Because Harry Potter is about - guess what - Harry F*cking Potter, so who else would beat Voldemort? Why is Anna Karenina the only one in Anna Karenina jumping in front of the freaking train? Quite the same reason why Frodo Baggins was the one to destroy the One Ring and Anakin Skywalker is the one to kill the Emperor and Juliet to get shagged سے طرف کی Romeo AND Harry Potter to beat آپ Know Who. Maybe I'm mistaken, but it's not Dudley Dursley who has a bit (maybe his nose) of Voldemort in his chest, so it can't quite be him, can it? Furthermore, if Dumbledore's dead, Moody is dead, Marauders are dead, Weasleys are runner ups, who do آپ think is the last Kick پچھواڑے, گدا standing? I'm telling you. But only whisper, so other people won't hear. It's Harry Potter.
The book is about him! Why does Bella have to be the one who ends up with Eddy? Why not Jessica? :)
Well, to answer your سوال مزید nicely ..
I think anyone could kill LV but in the prophecy it stated that Harry has power that Lord Voldemort doesn't have and would be his downfall. That's why he spent his time trying to kill Harry and so Harry became the "chosen one". Instead of waiting to get killed, he decided that he would fight him because the wizarding world's future was up to him.