Final answer:
Evidence of earlier coastal migrations in North America has not been discovered mainly due to rising sea levels at the end of the Pleistocene, which submerged potential archaeological sites. Later evidence on higher ground and South American pre-Clovis sites support the Coastal Migration Theory.
Step-by-step explanation:
The reason why evidence of earlier coastal migrations down the west coast of North America has not been discovered is largely due to rising sea levels that occurred at the end of the Pleistocene, which submerged the coastlines where the early migrants would have traveled or settled. These obscured sites are challenging to explore because they are now underwater. It is also supported by the fact that later coastal travel evidence has been found on higher ground in areas like Alaska and Canada, suggesting that earlier evidence may also be submerged. Limited research funding can occasionally hinder extensive underwater archaeological exploration, but it is primarily the rising sea levels that are responsible for the lack of evidence of these early coastal migrations.
There is much debate about the settling of the Americas, including the Bering Land Bridge Theory and the Coastal Migration Theory. Notably, sites in South America such as Monte Verde and Taima-Taima predate the North American Clovis sites, supporting the coastal migration hypothesis, although much fewer in number. Furthermore, the hypothetical findings of 15,000-year-old rafts and canoes along the Pacific coast would significantly bolster the coastal migration theory, reshaping our understanding of early human migration in the Americas.