Why did the migration of humans from Africa to the rest of the world take place so slowly?

A better question might be, "What happened to all the earlier humans that left Africa?"

Humans have been leaving Africa for 2 million years. Our branch left Kenya 60 thousand years ago. There were some Neanderthals left from an earlier migration out of Africa, but that was about it.

Anyway, it took 10 thousand years to get to Australia. The food along that path was the same as it had been in Africa. Needless to say, life was dangerous and survival was difficult.

In order for humans to move north, they had to solve several problems: How could they obtain food during periods when no food was available? How could they stay warm in colder weather? They had to create warmer clothing and discover a way to kill different animals. They created better stone tools so they could kill bigger animals, the middle stone age. They started wearing animal skins.

At that point they became more successful than the Neanderthals.

Then they could spread.