**it happens.
Shit happens.
Marxist Version: It happens as a result of the relations and conflicts of socio/economic classes throughout history.
Capitalist Version: It happens due to the actions of bold and creative entrepreneurs guided solely by implacable market forces.
Religious Version: Because God wants it to happen, and he wants it to happen to us.
My version: All of the above, but mostly, a lot of random, blind chaotic chance.
As Han Solo says (I paraphrase): "I've been all over this galaxy, kid, and if there's one thing I've learned is that there is no one Force that's responsible for everything. It's all just a bunch of nonsense."