why did civilization take ages to develop?

A leisured class - one capable of developing the trappings of civilization - can only develop when basic needs of shelter, sustenance and protection are met by a large working class (usually slaves). Also, no great civilization can develop without a locus, a strategic place to settle with fertile soil, a good water supply and a pleasant climate.

When the Mediterranean seafarers landed in Egypt thousands of years ago, they found all that they needed plus an abundance of docile African nomadic hunters and gatherers who were easy to enslave.

Civilization was unable to develop early in Europe because the Ice Age isolated the tribes and when they did meet they were incredibly hostile to each other. When all of your time is spent providing for shelter, sustenance and protection for yourself and family, there is no time left to do anything else.

Migrating south, to a warmer climate, was possible at the end of the Ice Age, and whereas our very first ancestors trekked north by foot - following the animals they hunted - our later ancestors used boats to migrate south via the Mediterranean and lived off fish.

Egypt was the garden of Eden for the early settlers from Europe. No wonder they didn't want to leave! Agriculture developed quickly through the use of slaves, as did the building of solid structures.

Trade in slaves and excess food enriched the developing Egyptiac civilization to such an extent that the leisured class, the masters, were able to establish incredibly wealthy dynasties around which a middle class of artisans and courtiers quickly developed.

The pattern set by the early settlers in Egypt was to be followed century after century by new waves of colonizers in other lands who either enslaved the indigenous populations or took their own slaves with them.