Of course you could ask the same question about a number of other civilisations.
Australian Aboriginees for example or to a lesser extent American Indians.
Or you could ask why we progressed when we did.
Humans have been in our current form for 100,000 years
And yet we only have any real civilisations of note in the last 6-8 thousand years.
As has been pointed out Northern Europe was a slow starter, Chinese, Indian and Islamic Science made us look like savages really up to the start of the 17th Century.
I think it was movable type printing that really lit the fuse - all of a sudden ideas were flying about that couldn't be contained and we were off.
But dont forget there were great African civilisations that died - we don't hear much about them but you should check out:
http://en.wikipedia.org/wiki/Monomotapa
and especially
http://en.wikipedia.org/wiki/History_of_Ethiop ia