A changing climate
Homo erectus was in Europe in some numbers about a million years ago. But about 600,000 years ago the climate changed. It started to get very cold. The shift was gradual but the overall trend was of great cooling. With so much water being locked up in the ice caps caused the sea-level to drop and land bridges connected the various continents, so that eventually these people were able to spread into the Americas across the Bering Straits and down the island chains of Indonesia towards New Guinea and Australia.